WPDB: Always use `mysqli` when available.

This change makes WordPress use `mysqli` on PHP <5.5 when available, instead of only in PHP 5.5+ and development installs.

WPDB includes a fallback to `mysql` in the event that the database connection fails with `mysqli` so incompatibilities should be few and far between.

Fixes #42812 for trunk.

Built from https://develop.svn.wordpress.org/trunk@42388


git-svn-id: https://core.svn.wordpress.org/trunk@42217 1a063a9b-81f0-0310-95a4-ce76da25c4cd