[TASK] Improve a type annotation in the setup extension

Also add a missing type cast.

Used command:
> ./Build/Scripts/runTests.sh -s phpstanGenerateBaseline

Resolves: #97426
Releases: main, 11.5
Change-Id: I5a48c0737833792f99652cc54a90db9bcbe798b8
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74808


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>