ci: add Java 21 to matrix

[ricellis]

PR summary

ci: add Java 21 to matrix

PR Checklist

Please make sure that your PR fulfills the following requirements:

• [x] The commit message follows the Angular Commit Message Guidelines.
• [x] Tests for the changes have been added (for bug fixes / features) - version matrix change only
• [x] Docs have been added / updated (for bug fixes / features) - docs update to follow

PR Type

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] New tests
• [x] Build/CI related changes
• [ ] Documentation content changes
• [ ] Other (please describe)

What is the current behavior?

Tests run on Java 11 and 17.

What is the new behavior?

Tests run on Java 11, 17 and 21.

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

Draft until Java 21 image availability.

Error: Could not find satisfied version for SemVer '21'.