For my understanding: Are these (exact) versions only for testing or will they have to be updated every time new SapMachine builds are made available?

@RealCLanger I think these are the versions that will be used productively and should be subject of regular update. Note that the current community java buildpack is the only one written in Ruby, all the rest community buildpacks follow the same structure, for example python buildpack. @ramonskie please correct me if wrong

you are correct.
the pattern should be for each java-buildpack version should have x version. this way you can always revert if necessary. how the old java buildpack handled versioning was totally different from all the other buildpacks.
that said. we should create ci automation to update the versions regularly. we already do that for all the other buildpacks anyways. where currently prs will be created for reviews and for major version bumps a issue wil be created.
so it requires a bit more of manual work