[backport#15452] GetKeyBirthTimes should return key ids, not destinations

Summary:
https://github.com/bitcoin/bitcoin/pull/15452/commits/78e407ad0c26190a22de1bc8ed900164a44a36c3

Depends on D6403

Concludes backport of Core PR15452

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6404