WMF is switching over to the one-week deployment schedule, starting tomorrow!
https://www.mediawiki.org/wiki/MediaWiki_1.22/Roadmap#Schedule_for_the_deplo...
For this time, I'm recommending we keep test2 on the wmf5 branch of Wikibase etc.
We will need to run all tests tomorrow to make sure there is (unlikely) no compatibility issue with our branch and wmf6 core.
Then we deploy our wmf6 branch of Wikibase etc. on Monday to wikidata and test2. Next Thursday, the clients all get wmf6 branch of Wikibase.
Cheers, Katie