Add support for bzlmod + Bazel 8

[jchadwick-buf]

This is a full-rewrite of the WORKSPACE.bazel rules + a bzlmod MODULE.bazel implementation.

For both Bazel 7.x and Bazel 8.x, all of the following combinations will work:

# Legacy WORKSPACE mode
bazel build --noenable_bzlmod --enable_workspace //... && bazel test --noenable_bzlmod --enable_workspace //...

# Bzlmod-only mode
bazel build --enable_bzlmod --noenable_workspace //... && bazel test --enable_bzlmod --noenable_workspace //...

# Hybrid incremental migration mode
bazel build --enable_bzlmod --enable_workspace //... && bazel test --enable_bzlmod --enable_workspace //...

Some modules needed to be updated for Bazel 8.x compatibility. Also, some WORKSPACE code need to be retained for Bazel 7.x compatibility that could otherwise be omitted.

Gazelle is set up to function in bzlmod mode and can be invoked using bazel mod tidy.

I don't think we should need to substantially change CI except to test all of the possible configuration modes to ensure they do not break on accident.

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Jan 6, 2025, 10:26 PM

[rodaine]

How can we automate setting this value so cutting releases does not need to remember to set it?

[jchadwick-buf]

Per discussion: Let's restrict release tagging to a release workflow, and have it do the work of ensuring that the module version is consistent with the tag.