[net8.0] Disable legacy Xamarin. (#18438)

Support policy: https://dotnet.microsoft.com/en-us/platform/support/policy/xamarin.

> Xcode 14 SDKs (iOS and iPadOS 16, macOS 13) will be the final versions Xamarin will target.

.NET 8 will ship with Xcode 15 support, so legacy Xamarin can be
disabled on this branch.

Make.config

@@ -289,7 +289,7 @@ INCLUDE_TVOS=1
 INCLUDE_MACCATALYST=1
 INCLUDE_DEVICE=1
 INCLUDE_DOTNET_WATCHOS=
-INCLUDE_XAMARIN_LEGACY=1
+INCLUDE_XAMARIN_LEGACY=
 INCLUDE_HOTRESTART=1
 
 ENABLE_DOTNET=1