Force hunk storage migration to the modern format
Ref T8475. This forces installs to migrate hunks to the modern format.
We stopped writing to the legacy format a very long time ago (2+ years?) without issues.
This doesn't destroy any data. T8623 has guidance and I'll publish more changelog guidance.
Test Plan: Faked some legacy data and migrated it.
Reviewed By: chad
Maniphest Tasks: T8475
Differential Revision: https://secure.phabricator.com/D17039