Add `WelcomePageNotAcceptableHandlerMapping` which will return
an HTTP 406 status if a suitable welcome page is found but
cannot be accepted for the request.
An additional mapper is used so that we don't need to change
the order of the `WelcomePageHandlerMapping`. It's possible
that users may have additional root handler mappings
registered to run after the `WelcomePageHandlerMapping`
and we still need to respect those.
Fixes gh-35559
d7ff379fb8