[alerts] Increase bot quarantine time threshold to 20m
[alerts] Increase bot quarantine time threshold to 20m

I keep getting alerts for quarantined devices on rack4 and by the time I
click through to Swarming they're already running tasks again.

Change-Id: I5ec863565e4a2dcebee802d5fea66b9384c375ed
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/368360
Reviewed-by: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>
Commit-Queue: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot