Ensure that we don't offer http2 in ALPN

[aturon]

Unfortunately, hyper-rustls's ALPN behavior is based on which Cargo features are enabled. To better match C@E behavior, we should avoid offering ALPN at all. We could do this either by landing a configuration option in hyper-rustls, or by just pulling in the bits of hyper-rustls we need (which are fairly minimal) and customizing the behavior directly.