Bump Google.Protobuf from 3.12.2 to 3.15.0 in /Common/DotNettyCore

Bumps [Google.Protobuf](https://github.com/protocolbuffers/protobuf) from 3.12.2 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@v3.12.2...v3.15.0)

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

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

Common/DotNettyCore/DotNettyCore.csproj

@@ -19,7 +19,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Google.Protobuf" Version="3.12.2" />
+    <PackageReference Include="Google.Protobuf" Version="3.15.0" />
     <PackageReference Include="Libuv" Version="1.10.0" />
     <PackageReference Include="System.Collections.Immutable" Version="1.7.0" />
     <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="4.7.1" />