Fix Iterator example for PHP 8.1+

The newly introduced tentative return types require us to specify
return types for the method implementation.  Since `mixed` is only
available as of PHP 8.0.0, we use the `ReturnTypeWillChange` attribute
to suppress the deprecation for now.

This includes user note 127109 (well, at least the actionable part of
it).

language/predefined/iterator.xml

@@ -77,27 +77,29 @@ class myIterator implements Iterator {
         $this->position = 0;
     }
 
-    public function rewind() {
+    public function rewind(): void {
         var_dump(__METHOD__);
         $this->position = 0;
     }
 
+    #[\ReturnTypeWillChange]
     public function current() {
         var_dump(__METHOD__);
         return $this->array[$this->position];
     }
 
+    #[\ReturnTypeWillChange]
     public function key() {
         var_dump(__METHOD__);
         return $this->position;
     }
 
-    public function next() {
+    public function next(): void {
         var_dump(__METHOD__);
         ++$this->position;
     }
 
-    public function valid() {
+    public function valid(): bool {
         var_dump(__METHOD__);
         return isset($this->array[$this->position]);
     }