Only use semi-colons for NoWarn (#6935)

Author: ericstj

test/Directory.Build.props

@@ -18,7 +18,7 @@
       CS1591: Missing XML comment for publicly visible type or member 'Type_or_Member'
       CS1712: Type parameter 'parameter' has no matching typeparam tag in the XML comment on 'Type_or_Member' (but other type parameters do)
     -->
-    <NoWarn>$(NoWarn),1573,1591,1712</NoWarn>
+    <NoWarn>$(NoWarn);1573;1591;1712</NoWarn>
   </PropertyGroup>
 
   <PropertyGroup>