release 4.0.0

>*NOTE:*
v4.x will be PHP-8 only, v3.x continues PHP-7 support

* PHP 8 compatibility
	- Drop ext-propro support
		PHP 8 removes the object get/set API from the ZendEngine, which renders
		that extension dysfunctional. As a consequence, the header property of
		http\Message and derived classes cannot be modified in place, and thus
		by reference.