CLEANUP: quic: use task_new_on() for single-threaded tasks

This simply replaces a call to task_new(1<<thr) with task_new_on(thr)
so that we can later isolate the changes required to add more thread
group stuff.
diff --git a/src/xprt_quic.c b/src/xprt_quic.c
index 32eeab8..0e7949c 100644
--- a/src/xprt_quic.c
+++ b/src/xprt_quic.c
@@ -4442,7 +4442,7 @@
 static int quic_conn_init_timer(struct quic_conn *qc)
 {
 	/* Attach this task to the same thread ID used for the connection */
-	qc->timer_task = task_new(1UL << qc->tid);
+	qc->timer_task = task_new_on(qc->tid);
 	if (!qc->timer_task)
 		return 0;