Update google-protobuf to v3.21.7

What does this MR do and why?

This doesn't appear to have any Ruby-specific changes, but some security scanners have picked up on a vulnerability in the Java implementation.

Full diff: https://github.com/protocolbuffers/protobuf/compare/v3.21.5...v3.21.7

Relates to gitaly#4534 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.