NET6 Binding projects should warn/error when attempting to bind/bundle system frameworks

[chamons]

In https://github.com/xamarin/xamarin-macios/issues/15485 it was found that a customer did this:

  <ItemGroup>
	<ObjcBindingApiDefinition Include="ApiDefinition.cs" />
	<ObjcBindingCoreSource Include="StructsAndEnums.cs" />
	<NativeReference Include="/System/Library/Frameworks/ScriptingBridge.framework" Kind="Framework" />
  </ItemGroup>

while trying to work around https://github.com/xamarin/xamarin-macios/issues/15489.

We should warn or error when the native reference starts with "/System/Library/Frameworks/" as nothing good can come of this.

[chamons]

FYI @rolfbjarne