[Engineering] 1.31.0-wmf.23 MediaWiki train rollback

[Tyler Cipriani]

Hi all!

Wednesday Train update: we're currently running 1.31.0-wmf.23 on group0 
only, so we're a day behind schedule.

I decided to rollback based on a particularly noisy notice[0]. The right 
folks are already aware and working on it (thanks all!).

Reminder that we have a task for each train rollout to track (potential) 
blockers[1]. You can follow that task to track future progress.

Additional reminder that you can use the "Wikimedia MediaWiki versions"[2] 
tool on Toolforge to know which wikis have which version at any time.

Choo Choo,
-- Tyler

[0]. <https://phabricator.wikimedia.org/T188555>
[1]. <https://phabricator.wikimedia.org/T183962>
[2]. <https://tools.wmflabs.org/versions/>