GH-554 Add Java 21 to version matrix

[ate47]

Issue resolved (if any): #554

Description of this pull request:

Java 21

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Please check all the lines before posting the pull request:

• I've created tests for all my changes
• My pull request isn't fixing or changing multiple unlinked elements (please create one pull request for each element)
• I've applied the code formatter (mvn formatter:format on the backend, npm run format on the frontend) before posting my pull request, mvn formatter:validate to validate the formatting on the backend, npm run validate on the frontend
• All my commits have relevant names
• I've squashed my commits (if necessary)