Update grpcio-tools requirement in /dev_tools/requirements

Updates the requirements on [grpcio-tools](https://github.com/grpc/grpc) to permit the latest version.
- [Release notes](https://github.com/grpc/grpc/releases)
- [Changelog](https://github.com/grpc/grpc/blob/master/doc/grpc_release_schedule.md)
- [Commits](grpc/grpc@v1.59.0...v1.68.1)

---
updated-dependencies:
- dependency-name: grpcio-tools
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

dev_tools/requirements/deps/protos.txt

@@ -1,7 +1,7 @@
 # dependencies for proto generation - used by cirq-google
 
 # This bundles protoc 3.24.3, which we use for generating proto code.
-grpcio-tools~=1.59.0
+grpcio-tools~=1.68.1
 
 mypy-protobuf~=3.4