Promote MigrateToNewXMIdentifier to opt-out instead of opt-in in the future

[chamons]

In #5654 I added
MigrateToNewXMIdentifier which is a csproj option that changes how things are built to solve a [long standing issue(https://github.com/NuGet/Home/issues/2662)] with Full and nuget.

We need to get nugets approval that this fixes things, and then do additional validation before changing this to opt-out.

I've filed this issue so we don't forget.

[chamons]

Since this will all change in .NET 5 this fall, and NuGet/Home#2662 is closed, let's close this as well.