Future-proof a check for net5.0 or later

- avoid comparisons involving `$(TargetFramework)` in .targets files
  - fine to compare it with `''` or `$(DefaultNetCoreTargetFramework)`

Author: dougbu

eng/targets/ReferenceAssembly.targets

@@ -50,8 +50,8 @@
       Condition=" '$(TargetFrameworkIdentifier)' != '.NETFramework' ">
     <PropertyGroup>
       <_RefSourceFileTFM>$(TargetFramework)</_RefSourceFileTFM>
-      <_RefSourceFileTFM Condition="$(TargetFramework.StartsWith('netcoreapp'))">netcoreapp</_RefSourceFileTFM>
-      <_RefSourceFileTFM Condition=" '$(TargetFramework)' == 'net5.0' ">netcoreapp</_RefSourceFileTFM>
+      <_RefSourceFileTFM Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp' AND
+          $([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '5.0'))">netcoreapp</_RefSourceFileTFM>
       <_RefProjectFileTFM>$(TargetFramework)</_RefProjectFileTFM>
       <_RefProjectFileTFM Condition="'$(TargetFramework)' == '$(DefaultNetCoreTargetFramework)'">%24(DefaultNetCoreTargetFramework)</_RefProjectFileTFM>