testing/kopano-webapp: should switch to php8
According to https://github.com/Kopano-dev/kopano-core/commit/9b3bbd27ea63af180ce73a30a218d34b6e6535a4 it could run even on php81 but I'm not personally using it to check /cc @Thermi
Its going to become required before testing/php7 will be moved to unmaintained
Issue depends on #13461 (closed)