Add upgrade_path.yml to control required stops

The former upgrade-path.yml was hosted under the Support toolbox
upgrade-path project:
https://gitlab.com/gitlab-com/support/toolbox/upgrade-path/-/blob/f1088b130ffdd876961753de3f634c6fa38f78f0/upgrade-path.yml

We've decied to have this file as a SSoT under the gitlab-org/gitlab
project instead, thus we're moving it here.

This project currently has the main purposes:

- Identify all the required stops in an upgrade path that user might
  have to pass through when upgrade their application.
- The toolbox upgrade path tool uses this SSoT to populate the upgrade
  path tool, which is a helper for the users to quickly identify where
  they sit in the path.
- The release tools bot reads this file to identify when a new upgrade
  stop has been added, then open a merge request on Omnibus to add this
  information there.

Future usages of this file:

- The release tools bot will also read this file and update the Chart
  repo. (similar to what it does for Omnibus)
- The database team can do several automations withing the
  gitlab-org/gitlab project to guarantee schema validation, and proper
  migration squashing. The upgrade path is usually tied to long running
  background migrations. So when a reviewer identifies a long running
  background migration, they might wanna propose a required stop at that
  point.