chore(deps): bump @grpc/grpc-js from 1.8.12 to 1.8.15 in /proto (#1336)

Bumps [@grpc/grpc-js](https://github.com/grpc/grpc-node) from 1.8.12 to 1.8.15.
- [Release notes](https://github.com/grpc/grpc-node/releases)
- [Commits](https://github.com/grpc/grpc-node/compare/@grpc/grpc-js@1.8.12...@grpc/grpc-js@1.8.15)

---
updated-dependencies:
- dependency-name: "@grpc/grpc-js"
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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