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

[Ahajha]

See https://github.com/jvolkman/rules_pycross/issues/209

The old protobuf version still needs Python 3.8, so attempting to load it fails. I see there is already a PR here (https://github.com/bazel-contrib/rules_python/pull/3367) but this is a much smaller change for an isolated issue.

[gemini-code-assist[bot]]

Summary of Changes

Hello @Ahajha, I'm Gemini Code Assist[^1]! 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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[aignas]

Why is rules_pycross using python3.8? It would be great if you could a little bit explain the breakage to me here.

[Ahajha]

Rules_pycross isn't, it's just that it manifested there somehow. The issue is that the older version of protobuf is using 3.8, which was dropped in later versions.