feat: Upgrade to protobuf ^6.0.0

- Update protobuf dependency from ^5.1.0 to ^6.0.0
- Exclude grpc-web example from workspace (bazel_worker doesn't support protobuf 6.0.0 yet)

Author: tsavo-at-pieces

pubspec.yaml

@@ -7,7 +7,7 @@ publish_to: none
 
 workspace:
   - example/googleapis
-  - example/grpc-web
+  # - example/grpc-web  # Excluded: bazel_worker doesn't support protobuf 6.0.0 yet
   - example/helloworld
   - example/metadata
   - example/route_guide
@@ -29,7 +29,7 @@ dependencies:
   meta: ^1.16.0
   http: ^1.4.0
   http2: ^2.3.1
-  protobuf: ^5.1.0
+  protobuf: ^6.0.0
   clock: ^1.1.2
   web: ^1.1.1