requires-php

Proof of concept for WordPress.org readme.txt Requires PHP tag.

• Checks against the Plugin API v1.2 to return dot org readme.txt data.
• Unsets the update_plugins transient if the server version of PHP is less than the version defined in the Requires PHP tag.
• Exits the Add Plugin process if the server version of PHP is less than the version defined in the Requires PHP tag.
• If the Requires PHP tag is missing from readme.txt everything proceeds as usual without the benefit of a check.

Patches in place for #43986 and #43987