Prefer http2 over http/1.1 in alpn when http2 is activated

[magnuswatn]

Creating a discussion, as per the issue template.

When http2 is enabled httpcore2 sends ["http/1.1", "h2"] in the alpn extension, and thus signals preference for http/1.1 over http2. This will make some servers choose http/1.1 even when http2 is supported. Since http2 is opt in, I think it makes sense to prefer http2 when it is enabled (["h2", "http/1.1"])

Also, a when only http2 is enabled (http1=False, http2=True) the alpn will still include and prefer http/1.1, so some servers will still pick http/1.1 which causes a connection failure. The alpn should only include h2 here.

I can make a PR for this.