po-mode: Remove support for Emacs versions < 23.

po-mode: Remove support for Emacs versions < 23.

Reported by Julien Palard <julien@palard.fr>
at <https://savannah.gnu.org/bugs/?64748>.

* gettext-tools/emacs/po-mode.el (po-mode-version-string): Bump to 2.29.
(po-buffer-substring, po-match-string, po-with-temp-buffer, po-kill-new,
po-read-event, po-force-mode-line-update): Remove portability polyfills.
(easy-menu-define, format-time-string): Assume present as well.
* gettext-tools/emacs/po-compat.el: Update.