validation: Mark g_chainman DEPRECATED

Summary:
Completes backport (6/6) of core PR18698:
https://github.com/bitcoin/bitcoin/pull/18698/commits/fab6b9d18fd48bbbd1939b1173723bc04c5824b5

Depends on D8564.

Test Plan:

ninja

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8565