Move upgrade-path.yml location

What this MR does

Moves the SSoT of the upgrade-path.yml to https://gitlab.com/gitlab-org/gitlab/-/raw/master/config/upgrade_path.yml.

After several discussions with multiple team, we decided to keep the SSoT for the upgrade stops in the gitlab-org/gitlab project, as this is the project which effectively introduces the majority of the changes which will end up requiring an upgrade stop.

Related issues

Move upgrade path information to `gitlab-org/gi... (gitlab-org/gitlab#455239 - closed)