Turn server.consistency_check.value_checksum_verification.enabled off by
default on this release branch, so that the backported verification
only runs where an operator enables it.

- Default the setting to false.
- Check in TestCheckConsistencyValueChecksums that nothing is counted by
  default, and enable the setting before expecting failures.

Epic: none

Release note (ops change): The value checksum verification in
consistency checks is disabled by default. Enable it with SET CLUSTER
SETTING server.consistency_check.value_checksum_verification.enabled =
true.

Co-Authored-By: roachdev-claude <roachdev-claude-bot@cockroachlabs.com>