antrea-controller should be backward compatible with antrea-agent to allow graceful upgrade

[tnqn]

Describe the bug
In Antrea v0.9.3, the API group "networking" is renamed to "controlplane"(#1147). When upgrading from 0.8 to 0.9.3, the control plane communication will be disrupted partially until the whole upgrade is done because the old antrea-agents cannot communicate with the new antrea-controller. The worst case is that the NetworkPolicy/ AppliedToGroup/ AddressGroup update cannot take effect until the agent on this Node is updated.

To Reproduce
Upgrade from 0.8.0 to 0.9.3.
old antrea-agents will have below logs:

I0915 03:53:09.160537       1 networkpolicy_controller.go:401] Starting watch for AppliedToGroup
I0915 03:53:09.160573       1 networkpolicy_controller.go:401] Starting watch for AddressGroup
I0915 03:53:09.160772       1 networkpolicy_controller.go:401] Starting watch for NetworkPolicy
W0915 03:53:09.163861       1 networkpolicy_controller.go:404] Failed to start watch for AddressGroup: unknown (get addressgroups.networking.antrea.tanzu.vmware.com)
W0915 03:53:09.163897       1 networkpolicy_controller.go:404] Failed to start watch for NetworkPolicy: unknown (get networkpolicies.networking.antrea.tanzu.vmware.com)
W0915 03:53:09.163916       1 networkpolicy_controller.go:404] Failed to start watch for AppliedToGroup: unknown (get appliedtogroups.networking.antrea.tanzu.vmware.com)

Expected
antrea-controller should be backward compatible with antrea-agent to allow graceful upgrade.

Actual behavior
antrea-agent 0.8.0 cannot communicate with antrea-controller 0.9.3.

Versions:
Please provide the following information:

• Antrea version (Docker image tag). 0.9.3