log: Move block file log messages to debug category

Summary:

log: Move "Leaving block file [...]: [...]" log message to debug category

log: Move "Pre-allocating up to position 0x[...] in [...].dat" log message to debug category

This is a backport of core#21041

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12040