Move ::fCheckpointsEnabled into ChainstateManager

Summary:
This is a partial backport of core#25704
https://github.com/bitcoin/bitcoin/pull/25704/commits/fa43188d86288fa6666307a77c106c8f069ebdbe

Note that additional changes in setup_common.cpp and miner_tests.cpp were required to make miner_tests pass with checkpoints disabled.
Depends on D15813

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15814