Bump Google.Protobuf from 3.15.6 to 3.15.7

Bumps [Google.Protobuf](https://github.com/protocolbuffers/protobuf) from 3.15.6 to 3.15.7.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](https://github.com/protocolbuffers/protobuf/compare/v3.15.6...v3.15.7)

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

Shadowsocks.Protobuf/Shadowsocks.Protobuf.csproj

@@ -5,7 +5,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Google.Protobuf" Version="3.15.6" />
+    <PackageReference Include="Google.Protobuf" Version="3.15.7" />
   </ItemGroup>
 
 </Project>