release-25.4: roachtest: ignore flaky npgsql test

[bghal]

Backport 1/1 commits from #167474 on behalf of @bghal.

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The Npgsql.Tests.CommandTests(NonMultiplexing).Cancel_async_soft has
been timing out.

Epic: none
Fixes: #167069

Release note: None

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Release justification:

[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 OR fixes for serious issues. Non-production includes test-only changes, build system changes, etc. Serious issues are 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. Reference the approved ENGREQ ticket in the PR body (e.g., "Fixes ENGREQ-123").

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.

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[blathers-crl]

❌ PR #167480 does not comply with backport policy

Confidence: high
Explanation: The PR involves modifications to a production file (pkg/cmd/roachtest/tests/npgsql_blocklist.go), specifically in the roachtest area, which aligns with development tools according to the backport policy. These files are used for configuring and running system tests, which although directly impacting test environments rather than production code, are crucial in maintaining the quality of the release. However, the PR lacks a complete 'Release justification' which is necessary for exempting it from the backport policy's critical bug or feature flag requirements. The PR attempts to address a flaky test by modifying a blocklist, a change typically not associated with critical bugs or new feature flags. Therefore, it violates the backport policy due to the absence of a sufficient release justification.
Recommendation: Provide a complete and detailed release justification in the PR body that aligns with the backport policy or reconsider the backport.

❌ ENGREQ Check Failed: Please check one of the backport category checkboxes.

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[cockroach-teamcity]

This change is