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.