[NET 6 Attribute Conversion] ObjCRuntime (#14006)

* Removing COREBUILD for mellite

* Remove existing attributes

* Attribute Conversion

* Reverting removing if COREBUILD

Co-authored-by: tj_devel709 <antlambe@microsoft.com>

src/ObjCRuntime/PlatformAvailability.cs

@@ -289,10 +289,13 @@ namespace ObjCRuntime
 		const int sys2 = 1937339186;
 
 		// Deprecated in OSX 10.8 - but no good alternative is (yet) available
-#if !NET
-		[Deprecated (PlatformName.MacOSX, 10, 8)]
-#else
+#if NET
 		[UnsupportedOSPlatform ("macos10.8")]
+#if MONOMAC
+		[Obsolete ("Starting with macos10.8.", DiagnosticId = "BI1234", UrlFormat = "https://github.com/xamarin/xamarin-macios/wiki/Obsolete")]
+#endif
+#else
+		[Deprecated (PlatformName.MacOSX, 10, 8)]
 #endif
 		[DllImport ("/System/Library/Frameworks/Carbon.framework/Versions/Current/Carbon")]
 		static extern int Gestalt (int selector, out int result);

src/ObjCRuntime/SystemVersion.cs

@@ -28,7 +28,14 @@ namespace ObjCRuntime {
 		const int sys2 = 1937339186;
 
 		// Deprecated in OSX 10.8 - but no good alternative is (yet) available
+#if NET
+		[UnsupportedOSPlatform ("macos10.8")]
+#if MONOMAC
+		[Obsolete ("Starting with macos10.8.", DiagnosticId = "BI1234", UrlFormat = "https://github.com/xamarin/xamarin-macios/wiki/Obsolete")]
+#endif
+#else
 		[Deprecated (PlatformName.MacOSX, 10, 8)]
+#endif
 		[DllImport ("/System/Library/Frameworks/Carbon.framework/Versions/Current/Carbon")]
 		static extern int Gestalt (int selector, out int result);