probe-engine: update signal test endpoints

[hellais]

According to the signal devs, they made some DNS updates on November 7th and deprecated an old service endpoint: textsecure-service.whispersystems.org.

That endpoint is currently being probed by OONI, and it's understandably displaying an "Anomaly" result since they made that change.

Here's the current list of endpoints that Signal clients are currently using:

api.backup.signal.org
cdn.signal.org
cdn2.signal.org
cdn3.signal.org
cdsi.signal.org
chat.signal.org
contentproxy.signal.org
sfu.voip.signal.org
storage.signal.org
svr2.signal.org
ud-chat.signal.org
updates.signal.org
updates2.signal.org
uptime.signal.org

[bassosimone]

Merged! Will release $soon.