Refresh po.m4 from gnulib upstream
po.m4 should be refreshed from the gnulib upstream. Not a high priority at the moment as there's no functional changes to it.
Back in 2021 this was only a copyright year update. It still is, but now the year in upstream file is 2022, and several other .m4 files from gnulib are similarly affected (our copies have not been updated to 2022)