[TASK] Make the return type of GeneralUtility::tempnam more specific

Also adapt the callers accordingly. This resolves a few PHPStan warnings
and allows simplifying some code.

Used command:

> ./Build/Scripts/runTests.sh -s phpstanGenerateBaseline

Resolves: #97703
Releases: main, 11.5
Change-Id: I29b7278960a519dd7c5e0d3e2966b75933941a38
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74788


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