logging: move checkpoint log into VALIDATION debug category

Summary:
Checkpoint mismatch log messages are very common because of Bitcoin Cash and Bitcoin SV peers, and the word "ERROR" showing up just after starting a node confuses users.

Move these messages to a debug category.

Depends on D12043

Test Plan:
ninja all check-all

Start bitcoind and verify for a few minutes that the error messages are no longer printed.

Run bitcoind -debug=VALIDATION and check that the error messages are logged.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12030