Use gitlab-workhorse 0.7.11

Versions 0.7.9 and 0.7.10 crash on previously valid 'authBackend'
settings.

I don't know how many people are affected by this, 'normal' Omnibus
installations are not. (If they were gitlab.com would have stopped
working :) ). Still this is something we should not ship.
Unfortunately 8-11-stable uses 0.7.10 (but 8.11.0 does not). So I
think we need to put this in the next 8.11 patch release.

See merge request !5983