Rename Trigger condition Subscribed to SubscriptionReady

[grantr]

Describe the bug
The Subscribed condition name is confusing. From #2163:

User might feel subscribed is to indicate the status of subscriber.

Expected behavior
The condition name is clearly related to the Subscription object under the trigger (if one exists).

To Reproduce
https://github.com/knative/eventing/blob/master/pkg/apis/eventing/v1beta1/trigger_lifecycle.go#L34

Knative release version
0.13.0

Additional context
As recommended by #2163, rename it to SubscriptionReady in v1alpha1 and v1beta1.

/kind good-first-issue
/area api

[MIBc]

I can help.

[lionelvillard]

cool, thanks @MIBc!

/assign @MIBc