Don't advise XQuartz upgrades for system X11

Yeah, yeah. The XQuartz/X11 stuff is confusing.

Library/Homebrew/cmd/doctor.rb

@@ -976,6 +976,7 @@ end
   def check_for_latest_xquartz
     quartz = MacOS::XQuartz.version
     return unless quartz
+    return if MacOS::XQuartz.provided_by_apple?
 
     quartz = Version.new(quartz)
     latest = Version.new(MacOS::XQuartz.latest_version)

Library/Homebrew/macos/xquartz.rb

@@ -16,6 +16,12 @@ module MacOS::XQuartz extend self
     "2.7.4"
   end
 
+  def provided_by_apple?
+    [FORGE_BUNDLE_ID, APPLE_BUNDLE_ID].find do |id|
+      MacOS.app_with_bundle_id(id)
+    end == APPLE_BUNDLE_ID
+  end
+
   # This should really be private, but for compatibility reasons it must
   # remain public. New code should use MacOS::XQuartz.{bin,lib,include}
   # instead, as that accounts for Xcode-only systems.