docs: update gitaly naming convention
What
Update the naming convention for Gitaly.
Why
In https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/24155+ we
had to rethink how our Gitaly servers are provisioned and ended up
ditching the uid
idea since it became extremly hard to do single node
recovery.
Reference: https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/24155