Update google-protobuf to v3.21.7

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 https://gitlab.com/gitlab-org/gitaly/-/issues/4534

Changelog: changed