Consolidate protobuf maintenance tasks

[samuel-williams-shopify]

Summary

• Move protobuf update and generation into private root Bake tasks.
• Remove the exported xDS recipe and shell updater.
• Pin every upstream protobuf source revision.
• Use temporary repositories with automatic cleanup and fail on missing sources or generation errors.
• Remove the obsolete xds/ directory from packaged gem files.

Verification

• bundle exec bake test (90 examples, 179 assertions)
• bundle exec rubocop
• Documentation coverage: 121/121
• generate_protos regenerated 87 Ruby protobuf classes in an isolated copy and the generated classes loaded successfully.
• update_protos refreshed 115 definitions from pinned revisions in an isolated copy, regenerated 87 Ruby classes, and the generated classes loaded successfully.