[wallet] Deprecate the generate RPC method

Summary:
This is a partial backport of Core PR14468 : https://github.com/bitcoin/bitcoin/pull/14468/commits/c9f02955b2e9062808a9455c4ee7d52cf401eef5

Depends on D5970

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D5971