Update AuthorizedKeysCommand for GitLab.com deployment

We have this feature enabled on GitLab.com: https://docs.gitlab.com/ee/administration/operations/fast_ssh_key_lookup.html

In previous releases, this document asked you to copy and paste some random shell script onto the filesystem. More recent versions integrated the necessary command into the gitlab-shell repository and updated the documentation to refer to it.

If you use the old version, rake gitlab:geo:check outputs warning text. We're seeing this a lot during GCP migration failover attempts, but it's not a blocker.

Let's update our deployment to follow the new command at some point. This will allow us to benefit from a simpler deployment, and automatic updates to the script.