Automatic merge from submit-queue (batch tested with PRs 41931, 39821, 41841, 42197, 42195)

Adding legacy ABAC for 1.6

This is a fork of a previous [pull request](https://github.com/kubernetes/kubernetes/pull/42014) to include feedback as the original author is unavailable.

Adds a mechanism to optionally enable legacy abac for 1.6 to provide a migration path for existing users.