R1:9e82cfcc214b

Use utf8_general_ci for "sort" columns in old MySQL

Summary:
Fixes T7287. This trades off 4-byte character support for case insensitivity in these columns, which is a much better trade on the balance.

Also adds more warnings about old MySQL. Note that we already issue a warning when you run "storage adjust" (which I've made stronger) and already "strongly recommend" MySQL 5.5 or newer in the install documentation.

Test Plan:
- Ran `storage adjust --disable-utf8mb4` to go to old definitions, then ran…