www/horde-base: do not use with PHP 8.x

Note: a patch is available at
https://github.com/horde/Core/commit/1ec6fa6d150257a1eac64d6cc987a79656d6a0dc
It could be back-ported, but I have not tested enough all the different
applications to be sure that it would be sufficient for Horde to become
compatible with PHP-8.

PR: 261743
Approved by: maintainer’s time-out