Revert "Require Schleuder version 4.0"

This reverts commit c36fc6e5.
5 jobs for debian/bullseye/3.6.0 in 6 minutes and 31 seconds (queued for 3 seconds)
latest
Status Name Job ID Coverage
  Static
passed codespell #222616

00:00:21

 
  Test
passed ruby:2.5 #222617

00:05:57

passed ruby:2.6 #222618

00:06:09

passed ruby:2.7 #222619

00:05:56

passed ruby:3.0 #222620

00:03:28