Bump Google.Protobuf in /examples/RemoteActivate/Messages

Bumps [Google.Protobuf](https://github.com/protocolbuffers/protobuf) from 3.7.0 to 3.15.0.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.7.0...v3.15.0)

---
updated-dependencies:
- dependency-name: Google.Protobuf
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

examples/RemoteActivate/Messages/Messages.csproj

@@ -10,7 +10,7 @@
     <None Include="Protos.proto" />
   </ItemGroup>
   <ItemGroup>
-    <PackageReference Include="Google.Protobuf" Version="3.7.0" />
+    <PackageReference Include="Google.Protobuf" Version="3.15.0" />
     <PackageReference Include="System.ValueTuple" Version="4.5.0" />
   </ItemGroup>
   <ItemGroup>

src/Proto.Actor/Proto.Actor.csproj

@@ -8,7 +8,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Google.Protobuf" Version="3.21.0" />
+    <PackageReference Include="Google.Protobuf" Version="3.15.0" />
     <PackageReference Include="Grpc.Tools" Version="2.42.0" PrivateAssets="All" />
     <PackageReference Include="IsExternalInit.System.Runtime.CompilerServices" Version="1.0.0" Condition="'$(TargetFramework)' == 'netstandard2.1'" />
     <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="6.0.0" />