12.0.0 Support for TYPO3 12

Full Changelog: 5.2.0...12.0.0

• [FEATURE] Disable submit, if deletionperiod for this form is reached - thx to Stefan Busemann (in2code)
• [FEATURE] Allow to disable powermail_cleaner in in the Frontend
• [TASK] Improve wording - thx to Ulfried Hermann (Uni Weimar)
• [TASK] Port changes of V4 to V5 - thx to Stefan Busemann (in2code)
• [DOCS] Apply documentation of version 4 - thx to Stefan Busemann (in2code)