[backport#13756] wallet: rename g_known_wallet_flags constant to KNOWN_WALLET_FLAGS

Summary:
https://github.com/bitcoin/bitcoin/pull/13756/commits/129a5bafd9a3efa2fa16d780885048a06566d262

Partial backport of Core PR13756

Test Plan:

ninja

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6368