Restrict workaround to MSBuild 15.0 and above

src/csharp/Grpc.Tools/build/_protobuf/Google.Protobuf.Tools.targets

@@ -40,10 +40,11 @@
   </ItemGroup>
 
   <!-- Workaround for VS Project System bug: -->
-  <!-- Duplicated items in None itemgroup without Generator attribute prevents the items -->
-  <!-- in Protobuf itemgroup with Generator attribute from triggering design time build. -->
-  <ItemGroup Condition=" '$(DisableProtobufDesignTimeBuild)' != 'true' " >
-    <!-- Working around this by removing the duplicated items from None itemgroup. -->
+  <!-- Duplicated items in None itemgroup without Generator attribute prevents the items in Protobuf -->
+  <!-- itemgroup with Generator attribute from triggering design time build. Working around this by -->
+  <!-- removing the duplicated items from None itemgroup. The Remove attribute was introduced -->
+  <!-- in the .NET Framework 3.5, but was only supported inside targets until MSBuild 15.0. -->
+  <ItemGroup Condition=" '$(MSBuildVersion)' &gt;= '15.0' and '$(DisableProtobufDesignTimeBuild)' != 'true' " >
     <None Remove="@(Protobuf)" />
   </ItemGroup>