Update generate:migration command to deal with new migration structure

stubs/migrations/blank.stub

@@ -1,18 +1,13 @@
 <?php
 
-namespace {{namespace}}\Migration;
+use Illuminate\Database\Schema\Builder;
 
-use Flarum\Database\AbstractMigration;
+return [
+    'up' => function (Builder $schema) {
+        //
+    },
 
-class {{name}} extends AbstractMigration
-{
-    public function up()
-    {
+    'down' => function (Builder $schema) {
         //
     }
-
-    public function down()
-    {
-        //
-    }
-}
+];