Remove all the conditional code looking for php < 73

This issue removes code that only was being executed by php < 73 and it's 100% safe to do so because Moodle 3.11 and up require php 73, hence it was not executed ever.

Removed code includes:

• ldap_control_paged_result and ldap_control_paged_result_response (that were deprecated in php 73 and have been removed in php 80).
• conditional code in the session manager, where some hacks were needed for php < 73. Note that this removes the private function append_samesite_cookie_attribute() completely because it was doing nothing (first line was returning for PHP >= 73).
• Also removed the old session.hash_function ini setting because it was removed in php 71.

Kept code includes:

• The environmental check_igbinary322_version test has not been removed because it doesn't hurt (always returns "ok" for php 73 sites) and doing it would involve to backport the environment.xml file to 39 and 310. Instead, a note has been added to MDL-71747 in order to get rid of that check for 4.1 and up.