TASK-4170 introduce java version 21

[Wolf2323]

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• The main commit(s) reference the Fibery ticket via a TASK-NNNN prefix in the commit message subject
• Include a human-readable description of what the pull request is trying to accomplish
• The CI build passes

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• New automated tests have been added
• The changes are already covered by automated tests and have been adapted
• These manual test cases cover this change:
• Steps for the reviewer(s) on how they can manually QA the changes:
• This is a minor internal change; basic CI/CD coverage is enough
• This is an incomplete feature hidden behind feature flag:
• This is proof-of-concept / experimental code not for production / marked @Deprecated
• No (significant) changes to production code

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• Classes and public methods have documentation (that doesn't just repeat the technical subject in English)
• Logging is implemented to monitor feature usage and troubleshoot problems in production
• These ReWiki pages are affected by this change and will be adapted: