Prevent admins from attempting hashed storage migration on read only DB

What does this MR do?

I noticed this small potential for confusion while working on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8433. The warnings introduced in that MR prompt an admin to migrate to hashed storage for Geo. And that may cause them to try it on a Geo secondary, which will just error in a strange way.

It was noted that this problem exists not just for Geo secondaries, but for any GitLab instance with a read-only DB, so I'm fixing it in CE.

I am open to suggestions on the wording of the message.

What are the relevant issue numbers?

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