Hashed Storage: allow migration to be retried in partially migrated projects

What does this MR do?

Allows project.migrate_to_hashed_storage! to execute when there is still a partially migrated project (storage_version: 1).

Previously we were checking if the project had migrated to any project version and prevented re-running the upgrade code. But if the upgrade fails in the middle of it, there is no way to re-execute.

What are the relevant issue numbers?

gitlab-org/gitlab-ce#53700

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

Closes #53700 (closed)