Stabilize feature `serve-h2c`.

It has been tested on Grebedoc (Fly.io servers) and found to work
satisfactorily, though without any apparent benefit. It requires client
opt-in and so enabling it at all times is benign.

src/main.go

···
 		server := http.Server{Handler: handler}
 		server.Protocols = new(http.Protocols)
 		server.Protocols.SetHTTP1(true)
-		if config.Feature("serve-h2c") {
-			server.Protocols.SetUnencryptedHTTP2(true)
-		}
+		server.Protocols.SetUnencryptedHTTP2(true)
 		logc.Fatalln(ctx, server.Serve(listener))
 	}
 }