Change Zend namespace to Laminas (#1963)

Also ensure backwards compatibility for extensions that use the Zend framework but don't explicitly require it.

framework/core/src/Extend/Middleware.php

@@ -11,7 +11,7 @@ namespace Flarum\Extend;
 
 use Flarum\Extension\Extension;
 use Illuminate\Contracts\Container\Container;
-use Zend\Stratigility\MiddlewarePipe;
+use Laminas\Stratigility\MiddlewarePipe;
 
 class Middleware implements ExtenderInterface
 {