Re-order update steps in the 8.14 -> 8.15 upgrade guide
From the commit message:
1. Gems need to be installed before we can run the
gitlab:workhorse:install task
1. Update gitlab-shell after gitlab-workhorse since it's a manual step
that needs to be done in `/home/git/gitlab-shell`
Also remove unnecessary `cd /home/git/gitlab` in the installation
guide.
Closes #26014 (closed)