[TASK] Use `list` as return type annotation for `GU::*Explode` methods

The `GU::revExplode` and `GU::trimExplode` methods return lists of
elements. Thus, we can safely annotate them as `list` instead of `array`
to allow better static code analysis. The annotation for
`GU::intExplode` is not changed at the moment since this method behaves
differently.

Resolves: #99881
Releases: main, 11.5
Change-Id: I111bb5d9482c165f6cee81ab542d2048f3634c13
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77835


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>