remove is_safe

1. this appears to be a copy-pasta from ..._html
2. on Pimcore 10 / Symfony 5 this appears not to have had an effect
3. on Pimcore 11 / Symfony 6 this results in double quotes being present inside an HTML attribute which breaks rendering

src/Document/Twig/Extension/Form.php

@@ -29,9 +29,6 @@ public function getFunctions(): array
             new TwigFunction(
                 'valantic_form_json',
                 fn (string $name): string => $this->formService->buildJsonString($name),
-                [
-                    'is_safe' => ['html'],
-                ]
             ),
         ];
     }