Bump Google.Protobuf from 3.11.4 to 3.15.0 in /Orm/Xtensive.Orm.MySql

Bumps [Google.Protobuf](https://github.com/protocolbuffers/protobuf) from 3.11.4 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@v3.11.4...v3.15.0)

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

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

Author: dependabot[bot]

Orm/Xtensive.Orm.MySql/Xtensive.Orm.MySql.csproj

@@ -30,7 +30,7 @@
     </Compile>
   </ItemGroup>
   <ItemGroup>
-    <PackageReference Include="Google.Protobuf" Version="3.11.4" />
+    <PackageReference Include="Google.Protobuf" Version="3.15.0" />
     <PackageReference Include="MySql.Data" Version="8.0.19" />
     <PackageReference Include="System.Xml.XPath.XmlDocument" Version="4.3.0" />
   </ItemGroup>