Make ContainerSource v1beta1 the stored version and add a migration tool

[bharattkukreja]

Problem
In 0.17, we promoted ContainerSource 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
ContainerSource in v1beta1 and able to migrate

Additional context (optional)
#3426

[grantr]

Blocked until 0.17 release.

[nachocano]

/assign @nlopezgi

[nlopezgi]

/reopen
per #3895 (comment): we also need to update the CRDs to mark that we are storing v1beta1 now, so that a fresh install has that, not just the updated clusters.

[knative-prow-robot]

@nlopezgi: Reopened this issue.

In response to this:

/reopen
per #3895 (comment): we also need to update the CRDs to mark that we are storing v1beta1 now, so that a fresh install has that, not just the updated clusters.

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