SupportedOSPlatform assembly attributes are not getting generated for version less platform name

[buyaa-n]

SupportedOSPlatform assembly attributes are not getting generated for version less platform name

In dotnet/runtime#39989 (comment) we decided to rename MinimumOSPlatformAttribute into RemovedInOSPlatformAttribute and support version less platform name. We had renamed the attribute with #12775 but the version less attribute name support is missing

<PropertyGroup>
  <TargetFramework>net5.0-windows</TargetFramework>
</PropertyGroup>

Only works when adding a valid version

<PropertyGroup>
   <TargetFramework>net5.0-windows7.0</TargetFramework>
</PropertyGroup>

CC @adamsitnik @jeffhandley @terrajobst @wli3

[terrajobst]

@marcpopMSFT the fix needs to go into RC2. Can we prioritize this?

[sfoslund]

Fixed by #13507