chore: downgrade grpc plugin version

[JoeWang1127]

Downgrade the grpc plugin version to 1.3.0, matches the version in legacylibrarian Dockerfile.

We can avoid downloading a different version of dependency when running librarian generate.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.41%. Comparing base (e5b3a13) to head (93c3e8a).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4809   +/-   ##
=======================================
  Coverage   81.41%   81.41%           
=======================================
  Files         130      130           
  Lines       10860    10860           
=======================================
  Hits         8842     8842           
  Misses       1383     1383           
  Partials      635      635           

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[gemini-code-assist]

Code Review

This pull request downgrades the google.golang.org/grpc/cmd/protoc-gen-go-grpc dependency from v1.6.0 to v1.3.0 and updates the go.sum file. The review feedback indicates that this dependency downgrade lacks a necessary explanation, which is crucial for future maintainability and aligns with the repository's style guide. It is suggested to add a code comment to clarify the reason for the downgrade, specifically to avoid issues with newer versions requiring protoc-gen-go-grpc to be in the system PATH.

[julieqiu]

We should add a post-submit check which fails if the version of google.golang.org/grpc/cmd/protoc-gen-go-grpc changes and something breaks, similar to what we have for google-cloud-rust.

[JoeWang1127]

We should add a post-submit check which fails if the version of google.golang.org/grpc/cmd/protoc-gen-go-grpc changes and something breaks, similar to what we have for google-cloud-rust.

Filed #4811