[CE Backport] Prevent migrate_to_hashed task with read only database

What does this MR do?

Backports part of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8433

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/8289

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.