MDL-81456 core: Use DI for all hook access

Using DI for all hook access means that it becomes significantly easier
to mock hooks and callbacks for unit testing without fundamentally
altering the structure of the code purely for the purposes of unit
testing.

admin/tool/mfa/classes/manager.php

@@ -334,7 +334,7 @@ class manager {
 
             // Allow plugins to callback as soon possible after user has passed MFA.
             $hook = new \tool_mfa\hook\after_user_passed_mfa();
-            \core\hook\manager::get_instance()->dispatch($hook);
+            \core\di::get(\core\hook\manager::class)->dispatch($hook);
 
             // Add/update record in DB for users last mfa auth.
             self::update_pass_time();