Fetch the URL from the Broker.Status instead of hardcoding it for upgrade tests

[cardil]

Problem
PR #2388 introduce an upgrade tests. In those I utilize feeding events directly to a broker. At that time there was only ChannelBasedBroker. Now, it will be removed. We should fetch the URL of the broker from the Broker.Status field instead of hardcoding it in:

eventing/test/upgrade/prober/config.toml

Line 3 in f1a586b

address = 'http://default-broker.{{- .Namespace -}}.svc.cluster.local'

Persona:
Which persona is this feature for? Contributors

Exit Criteria
Done if the broker address is no longer hard coded in source code.

Time Estimate (optional):
4h

Additional context (optional)
As noticed by @vaikas in #2388 (comment)

[cardil]

/assign @cardil