gitlab-workhorse version should be update to 0.7.8 in our documentation

› cat GITLAB_WORKHORSE_VERSION  
0.7.8

› git gr '0\.7\.7'
doc/install/installation.md:    sudo -u git -H git checkout v0.7.7
doc/update/8.9-to-8.10.md:sudo -u git -H git checkout v0.7.7