Raise base version to java 17

[hazendaz]

Many projects are moving to java 17. Given we use jgit, it now requires java 17. We may want to release a final 4.6 now that is left at java 8. Then follow that by a java 11 with jgit 6. Then finally go to 7.0 on jgit.

4.6 -> 5.13 jgit last java 8
5.0.0 -> (get semantic) and jgit 6.10.x for java 11
6.0.0 -> jgit 7.0.x for java 17

If needed we can back port for those that need it but project is rather mature for what it is and I think due to underlying reasons of things moving to java 17, there might not be a strong use case but its easily done if needed.

[mathieucarbou]

@hazendaz I agree to give up java 8 yes like you said we should upgrade .
I also like your proposal for the versioning (bumping major digit).
Let's do that !