maintner: decrease poll interval from 15 to 5 min

When Gerrit emails are not sent, maintner polls Gerrit every
15 minutes. We can safely reduce this to every 5 min given the
relatively small number of bots using maintner and the current
quota of 0.2 qps per day that Gerrit permits for unauthenticated
requests.

Update golang/go#21192

Change-Id: I233bc50998927daedb34d81436e182579d2b4031
Reviewed-on: https://go-review.googlesource.com/57710
Reviewed-by: Sarah Adams <shadams@google.com>
diff --git a/maintner/gerrit.go b/maintner/gerrit.go
index 5934d5f..f34191d 100644
--- a/maintner/gerrit.go
+++ b/maintner/gerrit.go
@@ -818,7 +818,7 @@
 		if !loop {
 			return nil
 		}
-		timer := time.NewTimer(15 * time.Minute)
+		timer := time.NewTimer(5 * time.Minute)
 		select {
 		case <-ctx.Done():
 			timer.Stop()