chore(java-sdk): use java 21 in CI matrix

[jimmyjames]

Description

Use Java 21 (LTS) instead of Java 20 in CI matrix configuration.

References

• Java 21 released in September 2023

Review Checklist

• [x] I have clicked on "allow edits by maintainers".
• [ ] I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• [x] The correct base branch is being used, if not main
• [ ] I have added tests to validate that the change in functionality is working as expected