[BUGFIX] Deprecation free interfaces in FinisherVariableProvider

Set #[\ReturnTypeWillChange] and corresponding @todo comments
to methods of implemented interfaces in FinisherVariableProvider
to avoid deprecation messages without breaking compatibility.
Do that for 'mixed' returns, and use correct return types
directly for this @internal final class.

Resolves: #95755
Releases: master
Change-Id: Ic3d69e474419f48c58d4311b74047f952a483c29
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71961


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