Add back V1alpha1 policy report resources

[realshuting]

Related issue

Closes #2376.

Milestone of this PR

/milestone 1.4.3

What type of PR is this

/kind api-change

Proposed Changes

Add v1alpha1 policy report schema back to handle backward compatibility during upgrade.

Proof Manifests

With Kyverno v1.4.2 installed on the cluster:

✗ k get -n kyverno pod -o yaml | grep image:
              f:image: {}
              f:image: {}
      image: ghcr.io/kyverno/kyverno:v1.4.2
    - image: ghcr.io/kyverno/kyvernopre:v1.4.2
      image: ghcr.io/kyverno/kyverno:v1.4.2
      image: ghcr.io/kyverno/kyvernopre:v1.4.2

Apply the local changes to upgrade Kyverno to use latest tag:

✗ k get -n kyverno pod -o yaml | grep image:
              f:image: {}
              f:image: {}
      image: ghcr.io/kyverno/kyverno:latest
    - image: ghcr.io/kyverno/kyvernopre:latest
      image: ghcr.io/kyverno/kyverno:latest
      image: ghcr.io/kyverno/kyvernopre:latest

The pod is running and the policy report is with version v1alpha2:

✗ k get pod -n kyverno
NAMESPACE            NAME                                         READY   STATUS    RESTARTS   AGE
kyverno              kyverno-698c7fdf97-lk89p                     1/1     Running   0          46s

✗ k get polr polr-ns-default
NAME              PASS   FAIL   WARN   ERROR   SKIP   AGE
polr-ns-default   0      0      0      0       0      11m

✗ k get polr polr-ns-default -o yaml
apiVersion: wgpolicyk8s.io/v1alpha2
kind: PolicyReport
metadata:
  creationTimestamp: "2021-09-10T00:08:11Z"
  generation: 2
...

Checklist

• I have read the contributing guidelines.
• [] I have added tests that prove my fix is effective or that my feature works.
• [] My PR contains new or altered behavior to Kyverno and
  • [] I have added or changed the documentation myself in an existing PR and the link is:
  • [] I have raised an issue in kyverno/website to track the doc update and the link is:
  • [] I have read the PR documentation guide and followed the process including adding proof manifests to this PR.

Further Comments

cc @fjogeleit