Use proper Maven targets rather than legacy compat ones

[Kernald]

Using legacy targets forces downstream users to enable compat repositories as well.

[jamesward]

The Bazel tests are failing with the same issues I ran into & fixed in #319. If you rebase this on #319, that'd be awesome. Otherwise, you can copy & paste / cherry-pick any of my changes and then I'll rebase on yours. :)

[Kernald]

I was actually about to submit another PR fixing Bazel version to 4.2.2. While updating grpc-java is the proper solution, fixing the version we use (for now to 4.2.2, or to 5.0. once #319 is merged) is probably a good thing moving forward anyway :-)

[Kernald]

#322 is adding a .bazelversion, I'm happy to update it to 5.0.0 and wait for #319 to be merged.

[jamesward]

Thank you!