refactor: replace doom-enlist with ensure-list

doom-enlist is now a deprecated alias for ensure-list, which is built
into Emacs 28.1+ and is its drop-in replacement. We've already
backported it for 27.x users in doom-lib (in 4bf4978).

Ref: 4bf49785fd

lisp/lib/buffers.el

@@ -160,7 +160,7 @@ See `doom-real-buffer-p' for details on what that means."
   "Return a list of buffers whose `major-mode' is `eq' to MODE(S).
 
 If DERIVED-P, test with `derived-mode-p', otherwise use `eq'."
-  (let ((modes (doom-enlist modes)))
+  (let ((modes (ensure-list modes)))
     (cl-remove-if-not (if derived-p
                           (lambda (buf)
                             (apply #'provided-mode-derived-p