Fix migration for executable zipfile

By doing the ALTER on the connection directly Doctrine didn't recognize it and re-added the column. We need to use `addSql`.

Author: nickygerritsen

webapp/migrations/Version20210407120356.php

@@ -63,7 +63,7 @@ public function up(Schema $schema) : void
                 );
             }
 
-            $this->connection->executeStatement('ALTER TABLE `executable` DROP COLUMN `zipfile`');
+            $this->addSql('ALTER TABLE `executable` DROP COLUMN `zipfile`');
         }
     }