po-mode: Fix incompatibility with the forthcoming Emacs 29.
Patch by Takesi Ayanokoji <ayanokoji.takesi@gmail.com>.
* gettext-tools/emacs/po-mode.el (po-font-lock-keywords): Use
font-lock-constant-face, introduced in Emacs 20.3, instead of the
long-deprecated font-lock-reference-face.
* NEWS: Mention it.
