fix: use 'auto' as default protocol for quick tunnels

Quick tunnel was hardcoding 'quic' as the default protocol when no
--protocol flag was specified, ignoring the documented default of 'auto'.

The Cloudflare docs state that 'auto' is the default:
https://developers.cloudflare.com/cloudflare-one/networks/connectors/cloudflare-tunnel/configure-tunnels/run-parameters/#protocol

This caused quick tunnels to always use QUIC even in environments where
HTTP/2 would be preferable or where QUIC is blocked.

Fixes #1609.

cmd/cloudflared/tunnel/quick_tunnel.go

@@ -84,7 +84,7 @@ func RunQuickTunnel(sc *subcommandContext) error {
 	}
 
 	if !sc.c.IsSet(flags.Protocol) {
-		_ = sc.c.Set(flags.Protocol, "quic")
+		_ = sc.c.Set(flags.Protocol, "auto")
 	}
 
 	// Override the number of connections used. Quick tunnels shouldn't be used for production usage,