http2 alpn (#6815)

## Problem

Proxy already supported HTTP2, but I expect no one is using it because
we don't advertise it in the TLS handshake.

## Summary of changes

#6335 without the websocket changes.

proxy/src/serverless.rs

@@ -88,7 +88,10 @@ pub async fn task_main(
             return Ok(());
         }
     };
-    let tls_acceptor: tokio_rustls::TlsAcceptor = tls_config.to_server_config().into();
+    let mut tls_server_config = rustls::ServerConfig::clone(&tls_config.to_server_config());
+    // prefer http2, but support http/1.1
+    tls_server_config.alpn_protocols = vec![b"h2".to_vec(), b"http/1.1".to_vec()];
+    let tls_acceptor: tokio_rustls::TlsAcceptor = Arc::new(tls_server_config).into();
 
     let mut addr_incoming = AddrIncoming::from_listener(ws_listener)?;
     let _ = addr_incoming.set_nodelay(true);