fix: Bump minimum protobuf version to 31.0 to fix dropped 3.8 support

[Ahajha]

See jvolkman/rules_pycross#209

The old protobuf version still needs Python 3.8, so attempting to load it fails. I see there is already a PR here (#3367) but this is a much smaller change for an isolated issue.

[gemini-code-assist]

Summary of Changes

Hello @Ahajha, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical compatibility issue by upgrading the protobuf dependency to a newer version. The previous protobuf release no longer supported Python 3.8, leading to runtime errors. By bumping the version to 31.0-rc1, this change ensures continued compatibility with Python 3.8 environments, providing a targeted fix for the reported problem.

Highlights

• Protobuf Version Update: The protobuf dependency has been updated from version 29.0-rc2 to 31.0-rc1 in the MODULE.bazel file.
• Python 3.8 Compatibility Fix: This change specifically addresses an issue where the older protobuf version dropped support for Python 3.8, causing failures when attempting to load it.

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[gemini-code-assist]

Code Review

This change correctly bumps the protobuf version to address the Python 3.8 support issue, which is a good targeted fix. While reviewing, I noticed a related point in examples/bzlmod/py_proto_library/BUILD.bazel. There is a test named external_import_test which is marked as manual with a TODO comment: reenable when com_google_protobuf is upgraded. Since this PR upgrades com_google_protobuf, it would be a great addition to re-enable this test and ensure it passes with the new protobuf version. This would increase confidence in the upgrade.

[gemini-code-assist]

For better maintainability, it would be helpful to add a comment explaining why this specific release candidate version of protobuf is being used. This provides context for future developers who might wonder about this version pin. Refactoring to a multi-line bazel_dep call would be a clean way to include this information.

bazel_dep(
    name = "protobuf",
    # Pinned to 31.0-rc1 to fix an issue with dropped Python 3.8 support.
    # See: https://github.com/jvolkman/rules_pycross/issues/209
    version = "31.0-rc1",
    repo_name = "com_google_protobuf",
)