Consolidate Dependabot NuGet fixes into central package update

[Copilot]

This PR consolidates the reported Dependabot updates into a single change set by updating centrally managed NuGet versions in AzureCertTools. The goal is to clear dependency alerts without broad code churn.

• Dependency baseline updates (central package management)

  • Updated Azure.Security.KeyVault.Certificates from 4.8.0 to 4.9.0.
  • Updated Microsoft.NET.Test.Sdk from 17.13.0 to 18.8.1.
  • Applied in one place: src/AzureCertTools/Directory.Packages.props, so all AzureCertTools projects inherit the same versions.

• Scope and impact

  • No production code or test logic changes.
  • Change is limited to package version resolution for AzureCertTools projects and tests.

<!-- src/AzureCertTools/Directory.Packages.props -->
<PackageVersion Include="Azure.Security.KeyVault.Certificates" Version="4.9.0" />
<PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.8.1" />