migrator: Use store factory in oobmigration registration

Created by: efritz

This is a refactor step towards #39578 (closed). This PR uses the new storeFactory parameter passed by the migrator service to the oobmigration registration function. This ensures that the migrator is operating data+schema migrations over a single set of connections rather than having two separate dials per schema (which causes duplicate prometheus metric registration issues).

Test plan

Existing CI pipelines.