owncloud 10.9.1 currently requires php-7.4 – (same will be true for the upcoming 10.10.0 release)
PHP 7.4 is still community supported, including security fixes.
My personal recommendation currently is to use either
a) Ubuntu 22.04 with ppa:/ondrej/php which brings you the latest php-7.4.29 with not only security, but also bugfixes.
b) Ubuntu 20.04 with php version 7.4.3-ubuntu2.10 which contains all known security fixes also present in 7.4.29
c) Or any other distribution which has their php at 7.4.x and applies security patches.