Use MSBuildVersion instead of MSBuildToolsVersion (#40349)

Fixes #40129

src/NuGet/Microsoft.NETCore.Compilers/build/Microsoft.NETCore.Compilers.props

@@ -5,7 +5,7 @@
   <Target Name="ValidateMSBuildToolsVersion" Condition="'$(BuildingProject)' == 'true'">
     <!-- The new editorconfig support requires MSBuild version 16.3. -->
     <Error Text="Microsoft.NETCore.Compilers is only supported on MSBuild v16.3 and above"
-           Condition="'$(MSBuildToolsVersion)' &lt; '16.3'" />
+           Condition="'$(MSBuildVersion)' &lt; '16.3'" />
   </Target>
 
   <!-- Always use the local build task, even if we just shell out to an exe in case there are

src/NuGet/Microsoft.Net.Compilers/build/Microsoft.Net.Compilers.props

@@ -12,7 +12,7 @@
   <Target Name="ValidateMSBuildToolsVersion" Condition="'$(BuildingProject)' == 'true'">
     <!-- The new editorconfig support requires MSBuild version 16.3. -->
     <Error Text="Microsoft.Net.Compilers is only supported on MSBuild v16.3 and above"
-           Condition="'$(MSBuildToolsVersion)' &lt; '16.3'" />
+           Condition="'$(MSBuildVersion)' &lt; '16.3'" />
   </Target>
 
   <!-- Always use the local build task, even if we just shell out to an exe in case there are