MDL-73424 general: Internal methods must have same type as parent

Otherwise the error is thrown in PHP8.1

backup/util/helper/backup_array_iterator.class.php

@@ -40,23 +40,25 @@ class backup_array_iterator implements iterator {
         $this->arr = $arr;
     }
 
-    public function rewind() {
-        return reset($this->arr);
+    public function rewind(): void {
+        reset($this->arr);
     }
 
+    #[\ReturnTypeWillChange]
     public function current() {
         return current($this->arr);
     }
 
+    #[\ReturnTypeWillChange]
     public function key() {
         return key($this->arr);
     }
 
-    public function next() {
-        return next($this->arr);
+    public function next(): void {
+        next($this->arr);
     }
 
-    public function valid() {
+    public function valid(): bool {
         return key($this->arr) !== null;
     }