chore: cleanup WORKSPACE

[aignas]

This is a task related to eventual cleanup of WORKSPACE support. This will happen when the minimum supported version is bazel 9, that is the first version that removes the WORKSPACE support.

Q: when should that happen? I personally think that having that done when bazel 9 is released and a stable bazel version no longer has WORKSPACE is a good time.

The following is a list of random things that we would need to do:

• Remove entry_points in whl_library.
• IS_BZLMOD_ENABLED has to go.
• TOOL_VERSIONS has to probably go and we can migrate the config to MODULE.bazel instead.