Build/Test Tools: Introduce `post-branching` Grunt task. #23320

Job	Run time
QUnit Tests / Run QUnit tests	2m 34s
Slack Notifications / Prepare notifications	6s
Slack Notifications / Success notifications	4s
Slack Notifications / Failure notifications	0s
Slack Notifications / Fixed notifications	0s
Slack Notifications / Cancelled notifications	0s
Failed workflow tasks	-1s
	2m 43s

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