[TASK] Deprecate TCA type=user without renderType

Using a custom renderType instead of a userFunc for
type=user TCA elements is much more flexible and suits
better into the overall FormEngine node construct.

The patch deprecates userFunc and outlines details on
how existing ones can be migrated over to custom renderTypes
without much hassle.

Resolves: #86163
Related: #86109
Releases: master
Change-Id: I2bad5f79dab5643efe7f8dc959e788b9426bfdd6
Reviewed-on: https://review.typo3.org/58198


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>