Fixing S.P.Corelib to not double include files and ensure that S.N.V is marked #nullable enable (#24526)
* Changing EnableDefaultCompileItems=false to EnableDefaultItems=false
* Taking the VS auto-fixes for the solution and corelib csproj
* Removing the duplicated T4 file includes in the main csproj
* Regenerating the included T4 templates
* Fixing S.P.Corelib to properly list available platforms/configurations
* Add a comment on the T4 template service
Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>