partially revert "some classes marked as final (BC break)"

This reverts commit 7dab8e3.

Author: dg

src/Forms/ControlGroup.php

@@ -15,7 +15,7 @@
 /**
  * A user group of form controls.
  */
-final class ControlGroup
+class ControlGroup
 {
 	protected \WeakMap $controls;
 	private array $options = [];