RPC: getpeerinfo: Deprecate "whitelisted" field (replaced by "permissions")

Summary:

If we were going to continue support for "whitelisted", we should have probably made it true if any permission flag was set, rather than only if "default permissions" were used.

This corrects the description, and deprecates it.

This is a backport of core#19770

Release notes are from https://github.com/bitcoin/bitcoin/blob/master/doc/release-notes/release-notes-0.21.0.md

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10500