Fix an issue with collation construction on "sort" columns for old MySQL

Summary:
Fixes T7422. We'll currently choose a "binary" charset with a "utf8_general_ci" collation on "sort" columns on older MySQL, which seems to be causing problems.

Choose "utf8" in this case instead.

(I attempted to simplify the logic, too, but that's the only actual change.)

Test Plan: Went back and forth with --disable-utf8mb4 on storage adjust, but this is version dependent so I'm not 100% sure it's the right fix.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7422

Differential Revision: https://secure.phabricator.com/D11928