[stubsabot] Bump protobuf to 5.28.*

Release: https://pypi.org/pypi/protobuf/5.28.0
Homepage: https://developers.google.com/protocol-buffers/
Repository: https://github.com/protocolbuffers/protobuf
Typeshed stubs: https://github.com/python/typeshed/tree/main/stubs/protobuf

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

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stubs/protobuf/METADATA.toml

@@ -1,6 +1,6 @@
 # Whenever you update version here, PROTOBUF_VERSION should be updated
 # in scripts/generate_proto_stubs.sh and vice-versa.
-version = "5.27.*"
+version = "5.28.*"
 upstream_repository = "https://github.com/protocolbuffers/protobuf"
 extra_description = "Partially generated using [mypy-protobuf==3.6.0](https://github.com/nipunn1313/mypy-protobuf/tree/v3.6.0) and libprotoc 26.1 on [protobuf v27.1](https://github.com/protocolbuffers/protobuf/releases/tag/v27.1) (python protobuf==5.27.1)."
 partial_stub = true