doctor checks XQuartz version

Closes Homebrew/homebrew#14182.

Library/Homebrew/cmd/doctor.rb

@@ -973,6 +973,20 @@ end
     EOS
   end
 
+  def check_for_latest_xquartz
+    quartz = MacOS::XQuartz.version
+    return unless quartz
+
+    quartz = Version.new(quartz)
+    latest = Version.new(MacOS::XQuartz.latest_version)
+
+    return if quartz >= latest
+
+    <<-EOS.undent
+      Your XQuartz (#{quartz}) is outdated
+      Please install XQuartz #{latest}.
+    EOS
+  end
 end # end class Checks
 
 module Homebrew extend self

Library/Homebrew/macos/xquartz.rb

@@ -12,6 +12,10 @@ module MacOS::XQuartz extend self
     end
   end
 
+  def latest_version
+    "2.7.4"
+  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.