Bump protobuf from 4.25.8 to 5.29.6 in /examples/grpc/plugin-python (#372)

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 4.25.8 to 5.29.6.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Commits](https://github.com/protocolbuffers/protobuf/commits)

---
updated-dependencies:
- dependency-name: protobuf
  dependency-version: 5.29.6
  dependency-type: direct:production
...

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Author: dependabot[bot]

examples/grpc/plugin-python/requirements.txt

@@ -1,3 +1,3 @@
 grpcio==1.59.0
 grpcio-health-checking==1.59.0
-protobuf==4.25.8
+protobuf==5.29.6