Define box line-width more carefully

Before Emacs 28, the value had to be a number.  In Emacs 30 it can
be a cons-cell of two numbers, but the manual says that it can still
be a number.  But that doesn't seem to work for all Emacs releases
in between, at least not when using a negative number.

Try to cater to the preferences of various Emacs releases.

lisp/transient.el

@@ -498,7 +498,7 @@ See info node `(transient)Enabling and Disabling Suffixes'."
   :group 'transient-faces)
 
 (defface transient-higher-level
-  `((t (:box ( :line-width -1
+  `((t (:box ( :line-width ,(if (>= emacs-major-version 28) (cons -1 -1) -1)
                :color ,(face-attribute 'shadow :foreground nil t)))))
   "Face optionally used to highlight suffixes on higher levels.
 Also see option `transient-highlight-higher-levels'."
@@ -560,13 +560,15 @@ character used to separate possible values from each other."
   :group 'transient-faces)
 
 (defface transient-nonstandard-key
-  '((t (:box (:line-width -1 :color "cyan"))))
+  `((t (:box ( :line-width ,(if (>= emacs-major-version 28) (cons -1 -1) -1)
+               :color "cyan"))))
   "Face optionally used to highlight keys conflicting with short-argument.
 Also see option `transient-highlight-mismatched-keys'."
   :group 'transient-faces)
 
 (defface transient-mismatched-key
-  '((t (:box (:line-width -1 :color "magenta"))))
+  `((t (:box ( :line-width ,(if (>= emacs-major-version 28) (cons -1 -1) -1)
+               :color "magenta"))))
   "Face optionally used to highlight keys without a short-argument.
 Also see option `transient-highlight-mismatched-keys'."
   :group 'transient-faces)