SDN-4900: clean up openshift-sdn references in the API

[danwinship]

• config.openshift.io:
  • Belatedly update a few places to now say that the default plugin is OVNKubernetes, not OpenShiftSDN.
  • Clarify the docs on NetworkMigration to make it clear that it is used for status, not spec.
• operator.openshift.io:
  • Remove references to OpenShift SDN being a supported network type.
  • Mark OpenShiftSDNConfig as deprecated.
  • Belatedly remove the comment claiming that OVNKubernetes is not supported. :grimacing:
  • Clarify that NetworkType migration is no longer supported by current versions of OpenShift, and so setting the associated fields will result in CNO rejecting the configuration.
• network.openshift.io
  • Deprecate all the (OpenShift-SDN-only) types.

Note that all of the network.openshift.io types are now neither read from nor written to by any OCP component. In theory, users might be using them for their own purposes for unknown reasons not related to openshift-sdn, but other than that there's no reason to not just remove those types completely. (Though this PR does not do that.)