Update Google.Protobuf package to close security vulnerability

Author: alex-kulakov

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>