Use new $required_php_version global

[nacin]

We created $required_php_version, so we might as well use it where we can. Already done for $required_mysql_version in [12480].

We're moving down the PHP check in wp-settings.php, but I don't think that should be a problem. Other option is to move up the WPINC define.

[nacin]

Forgot to mention in the ticket description that this patch also removes a check for at least 4.3 in wp-db.php.

[nacin]

Updated against trunk r12933 (changes to wp-settings.php were handled in #11881)

[nacin]

Patch refreshed.

[westi]

[12939] - Use $required_php_version. Props nacin. fixes #11637