Rename gitlab-monitor to gitlab-exporter

As part of https://gitlab.com/gitlab-org/gitlab-ce/issues/61704 we want to rename gitlab-monitor to gitlab-exporter, to be less confusing and better align with naming conventions in the community.

This PR is a mass Find and Replace of gitlab-monitor, gitlab-mon, GitlabMonitor, and so on.

I need to figure out:

• In which order do we merge the various MRs and actually rename the repo
• How to make the settings backwards compatible (eg for people having gitlab_monitor['...'] = ... in their configs
• What to do about the /var/log/gitlab/gitlab-monitor and /var/opt/gitlab/gitlab-monitor current default directories