Hi,
There is a thread where this question has been recently discussed : https://forum.dokuwiki.org/d/20480-error-in-igor-your-composer-dependencies-require-a-php-version-720. There was no clear conclusion, though...
After installing in a virtualbox VM an old Debian 10, Ondřej Surý's Debian repo for PHP, libapache2-mod-php8.0, and DokuWiki as per our instructions, modified to make sure php8.0 is installed, I got a perfectly working DokuWiki. I installed these packages :
apt-get install libapache2-mod-php8.0
apt-get install php8.0-xml php8.0-mbstring php8.0-zip php8.0-intl php8.0-gd
Although your PHP cli version is 8.0, it maybe that the php version used by your web server is still your old php 7.0 version. This is easy to check.
in the folder where doku.php is, create with a notepad a file named info.php
with this content :
<?php phpinfo();
Then point your browser to http://your-host/dokuwiki/info.php
What's the answer ?