[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>

Build/bamboo/src/main/java/core/PreMergeSpec.java

@@ -115,7 +115,7 @@ Plan createPlan() {
                 stageEarly,
                 stageMainStage
             )
-            .linkedRepositories("git.typo3.org Core 7.6")
+            .linkedRepositories("github TYPO3 TYPO3.CMS 7.6")
             .triggers(
                 new RepositoryPollingTrigger()
                     .name("Repository polling for post-merge builds"),