- Upgrade bazel
- Reduce the number of places where bazel version needs to be upgraded
in future.
- also make sure the list of bazel versions to test by bazelified tests
is loaded from supported_versions.txt (it was hardcoded before).
- ~~Try upgrading windows RBE build to bazel 6.3.2 as well.~~
The core idea:
- the source of truth for supported bazel versions is in
`bazel/supported_versions.txt`
- the first version listed in `bazel/supported_versions.txt` is
considered to be the "primary" bazel version and is going to be used in
most places thoroughout the repo.
- use templates to include the primary bazel version in testing
dockerfiles and in a newly introduced `.bazelversion` files (which gets
loaded by our existing `tools/bazel` wrapper).
~~Supersedes https://github.com/grpc/grpc/pull/33880~~