Upgrade for 12LTS with a view to version 13; publish a schedule

[vmans]

PDF is world wide the de facto format to use for invoices and other webshop related documents.

Upgrade for compatibility with 12LTS and with respect to the already known changes for version 13.

https://docs.typo3.org/c/typo3/cms-core/main/en-us/Changelog-12.html
https://docs.typo3.org/c/typo3/cms-core/main/en-us/Changelog-13.html

Publish a schedule, a plan of action.

[extcode]

Working currently on that. Tested in a TYPO3 v13.

[extcode]

Be free to test the branches 7.x and main.