Support Java 21

[bestbeforetoday]

As a Java application developer I want to build and run my application with Java 21 So that I can stay current with latest features and bug fixes

Java 21 LTS is scheduled for release on 2023-09-19. Once available, the following tasks are required:

• [x] Add Java 21 to GitHub Actions test matrix
• [x] Use Java 21 as the default Java version for version-agnostic GitHub Actions workflows
• [x] Either on next release or if no code changes are required to pass tests, update documentation to include Java 21 in compatibility matrix.