[BUGFIX] Apply AJAX scriptItems to FormEngine FlexFormSectionContainer

When resolving inline JavaScript invocations in issue #95954 it looked
like AJAX-aware elements in FormEngine would share a common abstract
implementation in the client-side handling. Actually AJAX response
handling is independent for each of those elements...

This change now applies changes, that previously have been made to
`TYPO3/CMS/Backend/FormEngine/InlineRelation/AjaxDispatcher`, to
`TYPO3/CMS/Backend/FormEngine/Container/FlexFormSectionContainer`.

Resolves: #96027
Releases: master, 11.5
Change-Id: Icf507a32399f72182597e9c8d650fb2d9a35e480
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72231


Tested-by: core-ci <typo3@b13.com>
Tested-by: Jörg Bösche <typo3@joergboesche.de>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Jörg Bösche <typo3@joergboesche.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>