Change build to support net5.0 TFs

This updates our build to support the use of `net5.0` as a target
framework inside of our repository.

Note: this is back porting some changes from the covariant returns
branch. Wanted to get them into master sooner so we get more exposure to
them.

eng/targets/Imports.targets

@@ -36,7 +36,7 @@
       This condition will be evaluated multiple times in multi-targeted projects hence need to be careful
       to only set in the inner builds, not the outer build where only $(TargetFrameworks) is defined.
     -->
-    <DisableNullableWarnings Condition="'$(DisableNullableWarnings)' == '' AND $(TargetFrameworks.Contains('netcoreapp3.1')) AND '$(TargetFramework)' != '' AND '$(TargetFramework)' != 'netcoreapp3.1'">true</DisableNullableWarnings>
+    <DisableNullableWarnings Condition="'$(DisableNullableWarnings)' == '' AND '$(TargetFramework)' != '' AND '$(TargetFrameworkIdentifier)' != '.NETCoreApp'">true</DisableNullableWarnings>
 
     <!--
       Disable code style analyzers in "older" targets for a multi-targeted project. These analyzers don't

src/Tools/BuildBoss/ProjectCheckerUtil.cs

@@ -274,6 +274,7 @@ private bool CheckTargetFrameworks(TextWriter textWriter)
                     case "net20":
                     case "net472":
                     case "netcoreapp3.1":
+                    case "net5.0":
                         continue;
                 }