Rebuilding the database schema on MySQL 5.7+ uses native json columns

[zerocrates]

Doctrine detects they're supported and uses them rather than just making a comment as it does for older versions.

The issue is that this get saved into the schema for install/update and would mean all users would need to be on MySQL 5.7+ (slash whatever is the equivalent in Maria terms).

Fixing this is necessary to not have to do a bunch of annoying workarounds whenever we want to change the database.