Test migration paths

## What does this MR do?

It checks out the code base at v8.0.5, seeds the database, and runs all migrations. 

## Are there points in the code the reviewer needs to double check?

If this might make sense, does this simulate enough what happens in the real world.

## What are the relevant issue numbers?

Closes #14549 

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4874