[stubsabot] Bump protobuf to 4.21.*

Release: https://pypi.org/pypi/protobuf/4.21.7
Homepage: https://developers.google.com/protocol-buffers/

If stubtest fails for this PR:
- Leave this PR open (as a reminder, and to prevent stubsabot from opening another PR)
- Fix stubtest failures in another PR, then close this PR

Note that you will need to close and re-open the PR in order to trigger CI

stubs/protobuf/METADATA.toml

@@ -1,2 +1,2 @@
-version = "3.20.*"
+version = "4.21.*"
 extra_description = "Generated with aid from mypy-protobuf v3.3.0"