[cv] change scaling type of listener from manual to auto
[cv] change scaling type of listener from manual to auto

The instances taking traffic will server the warm-up requests.
The target_cpu_utilization is 0.6 by default.

If listener gets overloaded, appengine will add more instances.
Appengine still sends SIGTERM to instances for graceful shutdown [1]
- subscribers should have enough time to nack all the buffered messages.
- subscribers stop pulling messages once the context is cancelled [2]
- once an instance stops receiving warm-up requests, it will soon get
shutdown.
[1]: https://cloud.google.com/appengine/docs/standard/how-instances-are-managed#shutdown
[2]: https://github.com/googleapis/google-cloud-go/blob/pubsub/v1.26.0/pubsub/subscription.go#L1131

Bug: 1358208
Change-Id: I14d1c7fb0b26dbf6b9341f62c6daab5bbc1d5619
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3991127
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Commit-Queue: Scott Lee <1dcb292530f82960316cb3d87cd67c3f212b79cb@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go