release-25.2: roachtest: deflake cdc/ledger

[blathers-crl]

Backport 1/1 commits from #156985 on behalf of @andyyang890.

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This patch deflakes cdc/ledger by increasing the target steady latency
from 1 minute to 3 minutes. It also eliminates a weird quirk where the
latency verifier expects the latency to be less than half of the target
latency before it'll be considered steady, which led to weird logs and
added unnecessary complexity.

Fixes #151381

Release note: None

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Release justification: test-only change

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes. Includes test-only changes, build system changes, etc.
• Fixes for serious issues. Defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to.

Add a brief release justification to the PR description explaining your selection.

Also, confirm that the change does not break backward compatibility and complies with all aspects of the backport policy.

All backports must be reviewed by the TL and EM for the owning area.

[blathers-crl]

✅ PR #157057 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: This pull request modifies test files within the 'pkg/cmd/roachtest/' directory, which are designated as non-production files according to the CockroachDB Backport Policy. The modifications in this PR adjust the test parameters and logic to improve the stability and reliability of the cdc/ledger test by adjusting its latency expectations. The 'Release justification: test-only change' directly stated in the PR body aligns with the policy exceptions for test-only changes, confirming that the PR need not meet the critical bug or feature flag requirements typically necessary for backporting. As only non-production files are affected, the change does not impact production database capabilities, system performance, or stability and thus remains backward compatible.

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[cockroach-teamcity]

This change is