Update php version to 8.1

From https://www.dokuwiki.org/requirements

PHP needs to be at least version 7.2 since the “Igor” release, support for previous PHP versions has been dropped on this release. PHP versions up to and including 7.4 are now EOL, therefore using the latest version of the 7.4.x branch or 8.x is highly recommended. Dokuwiki is compatible with PHP 8.0 and 8.1.