Hi Guys,
I managed to compile my app on all versions - 8.* and 9.* 
It may be useful to someone, so I’m sharing my pom version below.
Additionally:
- I’m using only Spring Scanner (I removed all <component-import… and <component… tags)
- I removed the use of all deprecated packages from Atlassian (only whole packages)
- I added some libraries to pom that are no longer available
pom.xml
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
... APP INFO ...
<dependencies>
<!-- CONFLUENCE 8.8 + 8.9 + 9.0 - START -->
<dependency>
<groupId>org.json</groupId>
<artifactId>json</artifactId>
<version>20240205</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>com.google.guava</groupId>
<artifactId>guava</artifactId>
<version>33.0.0-jre</version>
<scope>compile</scope>
<exclusions>
<exclusion>
<artifactId>jsr305</artifactId>
<groupId>com.google.code.findbugs</groupId>
</exclusion>
</exclusions>
</dependency>
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-text</artifactId>
<version>1.11.0</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>commons-codec</groupId>
<artifactId>commons-codec</artifactId>
<version>1.16.1</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>commons-fileupload</groupId>
<artifactId>commons-fileupload</artifactId>
<version>1.5</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>commons-io</groupId>
<artifactId>commons-io</artifactId>
<version>2.15.1</version>
<scope>compile</scope>
</dependency>
<!-- CONFLUENCE 8.8 + 8.9 + 9.0 - END -->
<!-- Provided - Other -->
<dependency>
<groupId>javax.ws.rs</groupId>
<artifactId>jsr311-api</artifactId>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>javax.servlet-api</artifactId>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>javax.inject</groupId>
<artifactId>javax.inject</artifactId>
<scope>provided</scope>
</dependency>
<!-- Provided - Atlassian -->
<dependency>
<groupId>com.atlassian.confluence</groupId>
<artifactId>confluence-rest-api</artifactId>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>com.atlassian.confluence.plugins</groupId>
<artifactId>confluence-space-ia</artifactId>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>com.atlassian.upm</groupId>
<artifactId>licensing-api</artifactId>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>com.atlassian.upm</groupId>
<artifactId>upm-api</artifactId>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>com.atlassian.templaterenderer</groupId>
<artifactId>atlassian-template-renderer-api</artifactId>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>com.atlassian.confluence</groupId>
<artifactId>confluence</artifactId>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>com.atlassian.plugin</groupId>
<artifactId>atlassian-spring-scanner-annotation</artifactId>
<version>${atlassian.spring.scanner.version}</version>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>com.atlassian.mail</groupId>
<artifactId>atlassian-mail</artifactId>
<scope>provided</scope>
</dependency>
</dependencies>
<dependencyManagement>
<dependencies>
<dependency>
<groupId>com.atlassian.confluence</groupId>
<artifactId>confluence-plugins-platform-pom</artifactId>
<version>${confluence.version}</version>
<type>pom</type>
<scope>import</scope>
</dependency>
<dependency>
<groupId>com.atlassian.platform.dependencies</groupId>
<artifactId>platform-public-api</artifactId>
<version>${platform.version}</version>
<type>pom</type>
<scope>import</scope>
</dependency>
<dependency>
<groupId>com.atlassian.platform.dependencies</groupId>
<artifactId>platform-deprecated-public-api</artifactId>
<version>${platform.version}</version>
<type>pom</type>
<scope>import</scope>
</dependency>
</dependencies>
</dependencyManagement>
<build>
<plugins>
<plugin>
<groupId>com.atlassian.maven.plugins</groupId>
<artifactId>confluence-maven-plugin</artifactId>
<version>${amps.version}</version>
<extensions>true</extensions>
<configuration>
<banningExcludes>
<exclude>org.apache.commons:commons-lang3</exclude>
<exclude>commons-fileupload:commons-fileupload</exclude>
<exclude>commons-io:commons-io</exclude>
<exclude>com.google.guava:guava</exclude>
</banningExcludes>
<productVersion>${confluence.version}</productVersion>
<productDataVersion>${confluence.data.version}</productDataVersion>
<compressResources>false</compressResources>
<enableQuickReload>true</enableQuickReload>
<skipRestDocGeneration>true</skipRestDocGeneration>
<allowGoogleTracking>false</allowGoogleTracking>
<skipManifestValidation>true</skipManifestValidation>
<instructions>
<Atlassian-Plugin-Key>${atlassian.plugin.key}</Atlassian-Plugin-Key>
<Spring-Context>*</Spring-Context>
</instructions>
<extractDependencies>false</extractDependencies>
</configuration>
</plugin>
<plugin>
<groupId>com.atlassian.plugin</groupId>
<artifactId>atlassian-spring-scanner-maven-plugin</artifactId>
<version>${atlassian.spring.scanner.version}</version>
<executions>
<execution>
<goals>
<goal>atlassian-spring-scanner</goal>
</goals>
<phase>process-classes</phase>
</execution>
</executions>
</plugin>
</plugins>
</build>
<properties>
<confluence.version>9.0.0-m13</confluence.version>
<platform.version>7.0.0-m20</platform.version>
<confluence.data.version>${confluence.version}</confluence.data.version>
<amps.version>8.13.5</amps.version>
<atlassian.spring.scanner.version>2.2.6</atlassian.spring.scanner.version>
<atlassian.plugin.key>${project.groupId}.${project.artifactId}</atlassian.plugin.key>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
<maven.compiler.source>8</maven.compiler.source>
<maven.compiler.target>8</maven.compiler.target>
</properties>
</project>
Cheers
Adam