TYPO3 12 support

[fnagel]

Update extension for TYPO3 CMS 12 LTS.

Release is planned for end of this summer. Hopefully. Any help is appreciated!

References

• https://get.typo3.org/release-notes/12.0.0
• https://docs.typo3.org/c/typo3/cms-core/main/en-us/Changelog-12.html
• https://notes.typo3.org/s/EjM2zA7Dr

[fnagel]

One can now use "Table garbage collection" (see https://docs.typo3.org/c/typo3/cms-scheduler/master/en-us/Installation/BaseTasks/Index.html) so the development of this extension has been stopped for now.