Switch back to the Emacs 28 pretty-printer function

If we're running Emacs 29+, use the pretty-printer function from Emacs
28 for consistency. We can remove this once we can guarantee that all
development will happen on Emacs 29+.

* el_client.el (xelb-parse): Force emacs-28-style pretty printing.

xelb-gen

@@ -179,6 +179,7 @@ an `xelb-auto-padding' attribute."
 (defun xelb-parse (file)
   "Parse an XCB protocol description file FILE (XML)."
   (let ((pp-escape-newlines nil)        ;do not escape newlines
+        (pp-default-function 'pp-28)    ;avoid unecessary churn
         result header)
     (with-temp-buffer
       (insert-file-contents file)