No need to change gitlab-workhorse version in the update guide

There will be no need to manually check the gitlab-workhorse version thanks to the new gitlab:workhorse:install task!

This should be merged only after gitlab-org/gitlab-ce!6574 is merged.

unmarked as a Work In Progress

@rspeicher Assigning to you since the dependent MR got merged! :)

assigned to @rspeicher

merged

mentioned in commit 5d9cc50d

@rymai Woop!

mentioned in commit test-nick/gitlab-ce@7d0d0376