Provide a way to consolidate deprecation warnings in logs

This is a followup to https://bugs.launchpad.net/oslo.policy/+bug/1836568

Keystone has proposed a change to their enforcer that eliminates most of the deprecation warnings from the logs[0] by logging a single message to notify the user that deprecated policies are present instead of logging each deprecated policy individually. As more projects adopt the scope-aware policy functionality, this is likely to be a thing that they will all need to keep their logs from being inundated. While it's too late to adopt this in Train because we're in feature freeze, I think in Ussuri we should make this an option (or not? maybe just make it the standard behavior?) in oslo.policy itself so every project doesn't have to reimplement the fix.

0: https://review.opendev.org/#/c/682798