Test against additional Java versions

[aabmass]

Right now the test matrix only tests with Java 11. We should expand this to test also 17 and 21. I tried doing this but ran into some issues with Mockito.

[psx95]

On a slightly related note, I think the current codebase fails to build on Java 21. So prior to adding the tests, the build needs to be fixed - it might also involve updating the current version of Gradle.