thank you very much for explanation 
My problems and doubts also resulted from this thread - Preparing for Confluence 9.0 - EAP coming soon - #142 by adam.labus
Additionally, I read https://developer.atlassian.com/platform/marketplace/dc-apps-platform-7/#moved-to-jakarta-dependencies and checked what was underneath and I managed to efficiently switch to the new jakarta.* dependencies in pom.xml (from Platform 7) as well as handle jakarta.xml.bind.Marshaller and Unmarshaller
Cheers
Adam