Remove v1alpha1 API

[a-thaler]

Description
Follow-up of introducing a new v1beta1 version in #767

Goal: Remove the deprecated version

Tasks:

• Assure that there as a removal announcement at least 3 month before
• Have metrics in conversion webhook to check how often conversion webhook has been invoked to know v1alpha1 invocations. If there is no real usage anymore, continue
• Remove obsolete CRD version v1alpha1

   versions:
   - name: v1beta1
     served: true
     storage: true

• Remove the old version v1alpha1 from the Status Subresource of the CRD status.storedVersions
• This should be handled via KLM as per OLM design doc Remove CRD version from the status.storedVersions if the version is not present in the CRD. lifecycle-manager#2905
• Remove the conversion webhooks