There is currently no solution other then a downgrade with:
composer selfupdate --1
Problem is that the Horde components we use for mail are only available via Hordes PEAR repository. Unfortunately Horde seems pretty dead currently, which is the reason they have not updated their components to Composer.
To support PHP 8.0 I already forked most, but not all, components yet.
My current plan is to fork them all and move with the new release to Composer 2.x.