Support Ingress and IngressClass v1

[pleshakov]

Proposed changes

• Use Ingress v1 in the code
• Reject Ingresses with resource backends, because the IC doesn't support such backends:

          - path: /icons
            pathType: ImplementationSpecific
            backend:
              resource:
                apiGroup: k8s.example.com
                kind: StorageBucket
                name: icon-assets

• Use IngressClass v1 in the code
• Remove k8s < 1.19 from nightly

Note: the existing tests work for k8s 1.19-1.21
in a separate PR we will update the tests so that they work for k8s 1.19-1.22

The PR doesn't change the minimum supported k8s version for the IC - that will be done separately.
The PR doesn't update any docs - that will be done separately.

[pleshakov]

(forced pushed to remove linting errors)