Increase minimum MySQL version to 5.5.5

[zerocrates]

Omeka Classic has basically forever had a minimum MySQL version of 5.0. Having a higher minimum lays the groundwork for using utf8mb4, added in the (also itself extremely old) MySQL 5.5 series.

5.5.5 specifically is:

• new enough to have utf8mb4
• the version MariaDB sometimes pretends to be, so this will conveniently avoid having to do different checking for Maria,
• the version Wordpress moved to requiring a few years ago

Increasing the minimum in the core, even without immediately adding migrations for the core tables to utf8mb4, allows plugins to safely migrate their own tables if they set the new version as the minimum. Some key plugins like Simple Pages and Exhibit Builder have "content" columns that are some of the more common places 4-byte UTF-8 characters might appear, and often ones that aren't indexed, avoiding index size compatibility concerns.

We'll have to account for the connection charset also... probably the way to go is to just replace any utf8 charset we see coming from db.ini with utf8mb4, so we're not relying on people updating their db.ini.