[TASK] Don't use GeneralUtility::getContainer in functional tests

As GeneralUtility::getContainer is marked internal, we do now avoid
to use this method in core tests in order to demonstrate best
practices for third party extensions that may use core tests as
inspiration.

This change requires an update for typo3/testing-framework which now
provides a getContainer() method as API in functional tests:

  composer require --dev typo3/testing-framework:^6.2.5

Releases: master
Resolves: #91363
Change-Id: I844973ddd3355d15c72307ac9533429333a396da
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64456


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Benni Mack <benni@typo3.org>