Do not validate nullable annotations when targeting net472

[tmat]

The code base currently uses ! suppression to avoid nullable warnings. Rather than littering the code with ! the nullability analysis should be disabled for net472 targets in projects that multi-target.

Alternatively, calls to APIs like string.IsNullOrEmpty that are annotated on net10 but not on net472 should be changed to null-annotated equivalents.

The editorconfig suppresses IDE0370: Suppression is unnecessary to allow unnecessary ! when not targeting net472. This should be removed.