Add role="dialog" in the modal via script

As [commented](https://github.com/twbs/bootstrap/pull/30755#issuecomment-626583855) by @patrickhlauke in #30755 

> We could even consider injecting the role="dialog" via scripting (the same way aria-modal="true" is being injected).

`role="dialog"` should be removed from the HTML markup and should be added in the `.modal` via script.