MDL-71011 php80: final private makes no sense, throwing warning

Ref: https://php.watch/versions/8.0/final-private-function#final-private

This applies #712 from upstream libraries.

Unrelated: I've also changed a occurrence of "private final"
by the correct (PSR-12) "final private" in the constructor
(the final&private combination is allowed in constructors)
in lib/classes/event/base.php, because it's the unique case
in core and phpcs was really insisting.

lib/classes/event/base.php

@@ -134,7 +134,7 @@ abstract class base implements \IteratorAggregate {
     /**
      * Private constructor, use create() or restore() methods instead.
      */
-    private final function __construct() {
+    final private function __construct() {
         $this->data = array_fill_keys(self::$fields, null);
 
         // Define some basic details.