[TASK] Switch from git.typo3.org to github for testing

typo3.org git/gerrit show flakiness lately. To not torture
the poor servers with our pesky testing so much we switch
the git clones to github/TYPO3/TYPO3.CMS and hope merges
are mirrored over there more quickly and they sustain our
testing load easily.

Resolves: #85606
Releases: master, 8.7, 7.6
Change-Id: I772d945a3bf697172cb26edb761f01e6cb8da4bf
Reviewed-on: https://review.typo3.org/57645


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>