I have no concluding answer to your question, but at https://developer.atlassian.com/platform/marketplace/dc-apps-platform-7/ it is listed that Confluence 9 is based on Plattform 7.0 (and I assume that the latest release will work since Atlassian uses Semantic Versioning). It also states that Confluence 8.8 is based on Platform 6.5.
But as far as I can tell, you need to adjust the versions of the platform POMs manually.
This post Platform 7 upgrade in progress: Learn about the first highlights and changes - #4 by clouless (and the whole thread Platform 7 upgrade in progress: Learn about the first highlights and changes) may provide more information on how we are supposed to use dependencies with Confluence 9 / Platform 7.