Don't fail CI builds on NuGet audit advisories

[horgh]

Summary

• Add a CI-only WarningsNotAsErrors entry for NU1901-NU1904 in the unit-test csproj, so NuGet audit advisories on dependencies don't red-X every build while we wait for a Dependabot upgrade.
• TreatWarningsAsErrors stays on, and local builds remain strict — the condition is keyed on the CI=true env var that GitHub Actions sets automatically. Advisories still appear in CI build output.

Context: run 24902042979 failed on OpenTelemetry.Exporter.OpenTelemetryProtocol 1.14.0 advisories. A Dependabot bump to 1.15.3 has since merged, but the CI failure blocked unrelated PRs in the meantime.

Test plan

• CI passes on this branch
• Introduce a known-vulnerable package locally (without CI=true) to confirm the build still fails locally

[gemini-code-assist]

Code Review

This pull request updates the unit test project configuration to treat NuGet audit warnings (NU1901-NU1904) as non-errors during CI builds, preventing build failures while allowing Dependabot to handle updates. Feedback suggests also including NU1905 for unknown severity levels and applying these settings to the main project or a shared configuration file to ensure consistent behavior across the entire solution.

[gemini-code-assist]

Consider including NU1905 in the list to cover vulnerabilities with an 'Unknown' severity level. Without it, the build will still fail if such an advisory is reported.

Additionally, this change is currently limited to the unit test project. Since the main library project (MaxMind.GeoIP2.csproj) also has TreatWarningsAsErrors enabled, vulnerabilities in its dependencies will still cause CI failures. To fully address the issue and prevent future build blocks, consider applying this configuration to all projects or moving it to a shared Directory.Build.props file.

    <!-- In CI, don't fail the build on NuGet audit advisories (NU1901-NU1905).
         Advisories still appear in build output, and Dependabot handles the
         actual fix. Local builds keep the strict behavior. -->
    <WarningsNotAsErrors Condition="'$(CI)' == 'true'">NU1901;NU1902;NU1903;NU1904;NU1905</WarningsNotAsErrors>