Fix LedgerHandle.batchReadUnconfirmedAsync: use slog log instead of LOG (#4782)

The batchReadUnconfirmedAsync method added in #4739 calls LOG.error(...),
but LedgerHandle was migrated to slog and only has a lowercase `log`
field. Master fails to compile.

Convert the call to the slog builder style used elsewhere in the file.

Author: merlimat