archwiki: update to 1.35.2-2
We should probably wait for https://phabricator.wikimedia.org/T281193 to be fixed first. Then we'll need to update the submodule in the archwiki repo and tag a new release.
It would also be nice to merge https://github.com/archlinux/archwiki/pull/32 and https://github.com/archlinux/archwiki/pull/38 in the mean time
As for the PHP 8 support: the MediaWiki 1.35.2 release notes still say only that:
MediaWiki 1.35 should mostly work on the recently released PHP 8.0, however it is not currently actively supported.
So we should probably stick with PHP 7 until there is "active" support for PHP 8. Note that switching from PHP 7 to PHP 8 is as easy as reverting d6320b75 (assuming that the PHP 7 leftovers can be cleaned up manually...)