This enables bzlmod by default so that, by default, builds are using it for both Bazel 6
and Bazel 7.

This exposed some gaps in our test coverage, so some new jobs are created to cover
minimum bzlmod and workspace versions. Basically, our CI config used mostly
workspace builds, while Bazel's downstream testing used bzlmod builds. Changing
our config to default to bzlmod means we lost most workspace build coverage.

Unfortunately, adding these new jobs goes over the CI job limit, so I consolidated
tests a bit:

• The py_proto_library example for bzlmod is moved under the bzlmod example. This saves
  about 6 jobs
• Change some tests with both (Bazel 7, workspace) and (Bazel 6, workspace) to have
  just one of the two. This saves about 4 jobs. Many jobs still test both, so coverage
  should still be sufficient.

Also some various cleanups:

• Deletes the last bazel-in-bazel integration test using our old, private, fork of
  bazel_integration_test. It hasn't been running in a long time and is broken.
• Renames the CI jobs to have shorter names.