Need a post-install job to clean up domain mapping webhook resources

[dprotaso]

When we merged the domain mapping webhook in https://github.com/knative/serving/pull/14082 older installations leave resources around.

The release notes didn't specify how clean up the resources. In theory we could create a k8s post-install job that'll do the deletion.

I think we should use a job because folks who use the operator don't do the manual steps