Track whitelist terminology in cobra etc.

The client is currently forced to use whitelist terminology due to dependencies

See:
https://github.com/knative/client/blob/b916a5b3ed356ad8fd0cbd91fbe511765c231bcf/pkg/kn/commands/options/options.go#L40
https://github.com/knative/client/blob/4a3cf245504a71eccec0b681bfc053c291e1cee2/cmd/kn/main_test.go#L163
https://github.com/knative/client/blob/8f7088f0c013f583d05abfe78325e177720c3ba3/cmd/kn/main.go#L76
https://github.com/knative/client/blob/5ed353d36b2a98e83d480e9167a46feeb9459004/pkg/kn/config/config.go#L107

There's already upstream proposals to change those, so let's make sure we consume once ready https://github.com/spf13/pflag/pull/261.

Start by checking the upstream spf13/pflag pull request #261, then inspect the cited locations in pkg/kn/commands/options/options.go, cmd/kn/main_test.go, cmd/kn/main.go, and pkg/kn/config/config.go. Once the dependency changes are available, update the client to consume them and verify that the referenced tests and terminology no longer require whitelist wording.

Written by the indexing model from the issue text.