[Engineering] Revert to 1.28.0-wmf.8

[Matthew Flaschen]

All wikis were rolled back to 1.28.0-wmf.8 last night due to an increase 
in https://phabricator.wikimedia.org/T119736 .  We're working on it, and 
will either go back to 1.28.0-wmf.9, or to 1.28.0-wmf.10 (at normal 
train pace) when it's resolved.

Matt