Fix PHP 8.1 return type deprecation notices.

src/Message/MessagePart.php

@@ -57,18 +57,27 @@ public function __construct(PartStreamContainer $streamContainer, IMimePart $par
         $this->observers = new SplObjectStorage();
     }
 
+    /**
+     * @return void
+     */
     #[\ReturnTypeWillChange]
     public function attach(SplObserver $observer)
     {
         $this->observers->attach($observer);
     }
 
+    /**
+     * @return void
+     */
     #[\ReturnTypeWillChange]
     public function detach(SplObserver $observer)
     {
         $this->observers->detach($observer);
     }
 
+    /**
+     * @return void
+     */
     #[\ReturnTypeWillChange]
     public function notify()
     {

src/Message/PartChildrenContainer.php

@@ -131,18 +131,27 @@ public function remove(IMessagePart $part)
         return null;
     }
 
+    /**
+     * @return bool
+     */
     #[\ReturnTypeWillChange]
     public function offsetExists($offset)
     {
         return isset($this->children[$offset]);
     }
 
+    /**
+     * @return mixed
+     */
     #[\ReturnTypeWillChange]
     public function offsetGet($offset)
     {
         return $this->offsetExists($offset) ? $this->children[$offset] : null;
     }
 
+    /**
+     * @return void
+     */
     #[\ReturnTypeWillChange]
     public function offsetSet($offset, $value)
     {
@@ -158,6 +167,9 @@ public function offsetSet($offset, $value)
         }
     }
 
+    /**
+     * @return void
+     */
     #[\ReturnTypeWillChange]
     public function offsetUnset($offset)
     {

src/Parser/Part/ParserPartStreamContainer.php

@@ -143,6 +143,9 @@ public function getStream()
         return parent::getStream();
     }
 
+    /**
+     * @return void
+     */
     #[\ReturnTypeWillChange]
     public function update(SplSubject $subject)
     {

src/Stream/MessagePartStream.php

@@ -59,6 +59,9 @@ public function __destruct()
         }
     }
 
+    /**
+     * @return void
+     */
     #[\ReturnTypeWillChange]
     public function update(SplSubject $subject)
     {