Removed Gitlab Upgrader found in /lib/gitlab/upgrader.rb

What does this MR do?

The Gitlab Upgrader found in lib/gitlab/upgrader.rb has been deprecated. Since the Upgrader has been deprecated, all of the code and documentation should be removed about it. From @ddavison help of isolating those changes, I was able to quickly go in and remove those files and lines.

At the moment, the only failed job in the Pipeline was the ee_compat_check (since there was no ee related branch for me).

What are the relevant issue numbers?

This addresses Issue #40270 (closed).

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #40270 (closed)