Convert bin/storage workflow to presume utf8mb4 as the default encoding

Summary: see comments on rP68904d941c54

Test Plan: run ,/bin/storage upgrade

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16370