Change storage to v1 for subscriptions

[aliok]

Fixes #3929

Proposed Changes

• Store subscriptions as v1
• Migration job config

Release Note

- 🧽 Update or clean up current behavior
Change storage version of remaining messaging.* resources from v1beta1 to v1:
          - "subscriptions.messaging.knative.dev"

action required: You must run pre-install job prior to upgrading to get messaging.* resources to v1 API.

Docs

No docs change necessary. The content at https://knative.dev/docs/install/upgrade-installation/#run-pre-install-tools-before-upgrade is good enough

[knative-test-reporter-robot]

The following jobs failed:

Test name	Triggers	Retries
pull-knative-eventing-conformance-tests	2020-08-27 10:26:25.177 +0000 UTC	1/3

Automatically retrying due to test flakiness...
/test pull-knative-eventing-conformance-tests

[matzew]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aliok, matzew

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