Promote Webhook support to GA upstream:

• Ensure we are ready to release with the current assumptions documented here  
• Remove the feature gate
• Add another means to select the certificate provider
  • Select cert-manager by default
• Update the downstream to select for the openshift-serviceca certificate provider
• Graduate any documentation to docs

A/C:

 - We agree that we are ready to GA upstream
 - Features Gate removed (for cert-manager and openshift-serviceca
 - Method of certificate provider selection developed and documented
 - Downstream OLM uses openshift-serviceca
 - Upstream OLM uses cert-manager by default
 - Upstream docs updated / graduated