Rework the generated TCA items for TYPO3 12LTS

[oliverklee]

https://docs.typo3.org/c/typo3/cms-core/main/en-us/Changelog/12.3/Deprecation-99739-IndexedArrayKeysForTCAItems.html

Configuration\FlexForms::buildFields()

Add a version check and version-specific unit tests (i.e., one test that gets executed in TYPO3 < 12, and one for >= 12).