Yes, for example in nginx config it is just one word in server definition. And if client doest support it, fallback automatically to standard https website. Really there is no reason not to enable it.
Use it? yes, major browsers speak it. Use only http2? Not a good idea.
My little knowledge tells me if the browser doesn't support http2 it will still work.
So what do you mean to not only use http2 ?
Wasn't quite clear what question was, so wanted to make sure you aren't asking if you can require support for HTTP2. Any normal setup will fall back to HTTP when the upgrade to HTTP2 doesn't work, yes.