Use `application/yaml`

[JosiahParry]

plumber/vignettes/rendering-output.Rmd

Line 64 in a3af795

`@serializer yaml` | `text/x-yaml` | Object processed with `yaml::as_yaml()`

The serializers show that the application/yaml mime type is not used when serializing yaml. The application/yaml mime type was registered in RFC 9512.

I think think it may make sense to use this new standard over the current text/x-yaml