Bump protobuf and protoc_plugin (#2021)

Bumps [protobuf](https://github.com/google/protobuf.dart) and [protoc_plugin](https://github.com/google/protobuf.dart). These dependencies needed to be updated together.

Updates `protobuf` 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)

Updates `protoc_plugin` from 20.0.1 to 21.0.0
- [Release notes](https://github.com/google/protobuf.dart/releases)
- [Commits](google/protobuf.dart@protoc_plugin-v20.0.1...protoc_plugin-v21.0.0)

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

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

pubspec.yaml

@@ -23,7 +23,7 @@ dependencies:
   package_config: ^2.0.0
   path: ^1.8.0
   pool: ^1.5.1
-  protobuf: ^2.0.0
+  protobuf: ">=2.0.0 <4.0.0"
   pub_semver: ^2.0.0
   source_maps: ^0.10.10
   source_span: ^1.10.0
@@ -46,7 +46,7 @@ dev_dependencies:
   grinder: ^0.9.0
   node_preamble: ^2.0.2
   lints: ^2.0.0
-  protoc_plugin: ^20.0.0
+  protoc_plugin: ">=20.0.0 <22.0.0"
   pub_api_client: ^2.1.1
   pubspec_parse: ^1.0.0
   test: ^1.16.7