[rpc] Deprecate getpeerinfo addnode field

Summary:
This field is now redundant since the connection type field will indicate
MANUAL for addnode connections.

This is a backport of core#19725 [4&5/5]
https://github.com/bitcoin/bitcoin/pull/19725/commits/50f94b34a33c954f6e207f509c93d33267a5c3e2
https://github.com/bitcoin/bitcoin/pull/19725/commits/a512925e19a70d7f6b80ac530a169f45ffaafa1c

Depends on D10373

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10374