[BUGFIX] Fix several type annotations in FormDefinition

This patches replaces several wrong or incomplete type annotations in
FormDefinition. Additionally, the `$renderables` property is now
explicitly declared and annotated as `array<int, Page>` since it only
contains pages.

Resolves: #97545
Releases: main, 11.5
Change-Id: I182a861276c65142be7266ec7b10420f8532372c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74508


Tested-by: core-ci <typo3@b13.com>
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74515


Tested-by: Elias Häußler <e.haeussler@familie-redlich.de>
Reviewed-by: Elias Häußler <e.haeussler@familie-redlich.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>