Use new $required_mysql_version global in wp-db.php

[nacin]

The wpdb class check for MySQL compatibility can now be converted to the new $required_mysql_version global. This also makes the string version-independent.

There was some concern about include order when updating the rest of core to use the new globals. This one's easy, since the function already uses $wp_version.

The remaining hard-coded PHP and MySQL version numbers are all manual i18n (or part of an external package) best I can tell. It might be nice to include version.php early enough to use it across those instances, but it won't help translators. And when the time comes to boost requirements, we all know how to use grep.

[nacin]

#11464

[westi]

(In [12480]) Use new $required_mysql_version global in wp-db.php when checking if the mysql version is new enough. Fixes #11478 props nacin.