[stubsabot] Bump protobuf to 4.21.*

Release: https://pypi.org/pypi/protobuf/4.21.9
Homepage: https://developers.google.com/protocol-buffers/

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Author: stubsabot

stubs/protobuf/METADATA.toml

@@ -1,2 +1,2 @@
-version = "3.20.*"
+version = "4.21.*"
 extra_description = "Generated with aid from mypy-protobuf v3.4.0"