Hashed Storage rollback mechanism

What does this MR do?

• Implements a rollback routine for the Hashed Storage to Legacy Storage.
• Removes the "(Experimental)" text from the settings page

Parts of this MR was extracted to:

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24128.
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24402.

Please merge them first.

What are the relevant issue numbers?

• #53966 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• 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

Related to #53966 (closed)