Make PingSource v1beta1 the stored version and add a migration tool

[lionelvillard]

Problem
In 0.17, we promoted PingSource to v1beta1.
This is to track that we should make v1beta1 the storage version for 0.18, and we should add a migration tool for it.
Similar to: https://github.com/knative/eventing/blob/master/config/pre-install/v0.16.0/storage-version-migration.yaml

Persona:
Which persona is this feature for?
Developer

Exit Criteria
PingSource in v1beta1 and able to migrate

Additional context (optional)
Prior "art": https://github.com/knative/eventing/pull/3274/files

[lionelvillard]

@cr22rc @aavarghese FYI.

[aavarghese]

/assign @aavarghese

[lionelvillard]

this is done.