Hello Maxx ,
you don't have to wait much longer: PHP 8.x support is released this week. However, grommunio only ships PHP modules for the versions shipped by the distribution. Debian 11 ships php 7.4: https://packages.debian.org/stable/php/php - Since grommunio ships an extra module, you have to take what the distribution provides or compile all for yourself.
If you want PHP 8.1 with DEB, you need to use Ubuntu 22.04 which has PHP 8.1 support built in.