[Intents] Adjust initial introduction to INPersonRelationship in macOS. (#13765)

Fixes:

    Introduced: 2 API with unneeded or incorrect version information
        [FAIL] 10.15 (Intents.INPersonRelationship Son) < 12.1 (Intents.INPersonRelationship)
        [FAIL] 10.15 (Intents.INPersonRelationship Daughter) < 12.1 (Intents.INPersonRelationship)

Fixes https://github.com/xamarin/maccore/issues/2540.

src/intents.cs

@@ -2229,7 +2229,7 @@ namespace Intents {
 		School,
 	}
 
-	[Mac (12, 1)]
+	[Mac (10, 15)] // headers say nothing, documentation says introduced macOS 12.1, but both headers and documents say some enum fields were available in macOS 10.15, so let's go with that for the enum itself as well.
 	[iOS (10, 2)]
 	[Watch (3, 2)]
 	[NoTV]