[BUGFIX] Deprecation free \ArrayAccess in FormRuntime

Set #[\ReturnTypeWillChange] and corresponding @todo comments
to methods of implemented interfaces in ext:form FormRuntime
to avoid deprecation messages without breaking compatibility.

We can however set some of the return values for @internal
marked methods.

Resolves: #95754
Releases: master
Change-Id: I76c52da760e9bc302884fddb9ad38852d3c13e6c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71959


Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>