Bump protobuf from 2.1.0 to 3.0.0 in /interop (#648)

Bumps [protobuf](https://github.com/google/protobuf.dart) from 2.1.0 to 3.0.0.
- [Release notes](https://github.com/google/protobuf.dart/releases)
- [Commits](google/protobuf.dart@protobuf-v2.1.0...protobuf-v3.0.0)

---
updated-dependencies:
- dependency-name: protobuf
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

interop/pubspec.yaml

@@ -11,7 +11,7 @@ dependencies:
   collection: ^1.14.11
   grpc:
     path: ../
-  protobuf: ^2.0.0
+  protobuf: ^3.0.0
 
 dev_dependencies:
   test: ^1.16.0