GenerateReferenceAssemblySource generates old license header

[ManickaP]

It should now generate only the new 2-line version introduces in: #38793
It still generates the old 3-line version.

[Dotnet-GitSync-Bot]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

Tagging subscribers to this area: @ViktorHofer
Notify danmosemsft if you want to be subscribed.

[stephentoub]

Being fixed by #38973