add flag to remove build warning

Author: akhera99

src/Workspaces/MSBuild/BuildHost/Microsoft.CodeAnalysis.Workspaces.MSBuild.BuildHost.csproj

@@ -21,6 +21,9 @@
          -->
     <PlatformTarget>AnyCPU</PlatformTarget>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+    
+    <!-- We suppress NETSDK1138 because we intentionally build against net6.0 to support a wider range of .NET SDKs -->
+    <NoWarn>$(NoWarn);NETSDK1138</NoWarn>
 
     <!--
       We specifically reference Microsoft.Build 17.3.4, since it's the oldest version still compatible $(NetRoslynBuildHostNetCoreVersion).