Force hunk storage migration to the modern format

Summary:
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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8475

Differential Revision: https://secure.phabricator.com/D17039