[TASK] Harden types in ext:dashboard

* Add some missing return types
* Remove wrong return type annotation
* Add missing import
* Use strict mode for `in_array`

Resolves: #90487
Releases: master
Change-Id: Iaadf0a1b38f743ac3777d5b3600584aa9fc957e7
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63369


Tested-by: Richard Haeser <richard@maxserv.com>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Richard Haeser <richard@maxserv.com>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>