don't emit attribute added diagnostic in strict mode (#28243)

Author: smasher164

src/ApiCompat/Microsoft.DotNet.ApiCompatibility/Rules/AttributesMustMatch.cs

@@ -62,6 +62,11 @@ private void AddDifference(IList<CompatDifference> differences, DifferenceType d
                 return;
             }
 
+            if (!_settings.StrictMode && dt == DifferenceType.Added)
+            {
+                return;
+            }
+
             CompatDifference difference = dt switch
             {
                 DifferenceType.Changed => new CompatDifference(