[backport#15670] refactor: combine Chain::findFirstBlockWithTime/findFirstBlockWithTimeAndHeight

Summary:
As suggested in #14711, pass height to CChain::FindEarliestAtLeast to
simplify Chain interface by combining findFirstBlockWithTime and
findFirstBlockWithTimeAndHeight into one

Extend findearliestatleast_edge_test in consequence

https://github.com/bitcoin/bitcoin/pull/15670/commits/765c0b364d41e9a251c3f88cbe203645854fd790

Depends on D6256

Backport of Core PR15670

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6257