po-mode: Avoid a compiler warning in Emacs 28.
Reported by Vincent Lefèvre <vincent@vinc17.net> via Santiago Vila
at <https://lists.gnu.org/archive/html/bug-gettext/2024-06/msg00031.html>.
* gettext-tools/emacs/po-mode.el (po-mode-version-string): Bump version.
(po-mode): Use write-contents-functions instead of write-contents-hooks.
