Move ::nMinimumChainWork into ChainstateManager

Summary:
This changes the minimum chain work for the bitcoin-chainstate
executable. Previously it was uint256{}, now it is the chain's default
minimum chain work
.
This is a partial backport of core#25704 and core#26409
https://github.com/bitcoin/bitcoin/pull/25704/commits/cccca83099453bf0882bce4f897f77eee5836e8b
https://github.com/bitcoin/bitcoin/pull/26409/commits/fa29ef00adac6f0842acdd38344820a1ce0e3087

Depends on D15812

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15813