Update new struct versioning rule (dotnet/coreclr#5429)

Update new struct versioning rule (dotnet/coreclr#5429)

If I understand this document, any change to a struct that invalidates its layout in other versioning bubbles would be considered a breaking change. I believe this would include adding or removing a field or changing the type of a field in an internal struct if that struct is included as a (possible non-public) field in a public struct; just because a struct is internal doesn't mean it's not subject to the new versioning rule.

Commit migrated from https://github.com/dotnet/coreclr/commit/ce99897cee0014833c2c472f00825d2db0f9605a