Modal - heading level prop

[jgiardino]

Currently, modals in core use <h1> as the heading level, but the react components have this set to <h3>.

There is some discussion related to what the recommended levels should be here:
w3c/aria-practices#551 (comment)

For this issue, we should provide a prop to allow consumers to change the level, and use a default value based on the recommendation by the aria-practices group.

For our default I'd suggest <h1> unless someone has a strong reason for <h2>.