Support was added in December 2020, unfortunately only weeks after your question @The-Evil-Fox. The only problem is it seems the Ubuntu repo is not up to date so if you install via apt install adminer you'll still be on a version that doesn't support PHP 8.
The easiest way I found to update is to still install it that way, but then use wget to download the latest version of adminer.php. So do something like this:
- Go to the adminer director
cd /usr/share/adminer
- Download the latest version of adminer:
sudo wget https://www.adminer.org/latest.php
- Delete adminer.php
- Then rename latest.php to adminer.php ->
sudo mv latest.php adminer.php
Done! That worked for me anyway... I'm not a Ubuntu pro though, so no idea why repo is so out of date.