Enable NuGet Audit for security vulnerability scanning

[Copilot]

NuGet Audit was previously disabled. This re-enables it to detect vulnerable packages during restore.

Changes

• Added explicit NuGet Audit configuration to Directory.Build.props:
  • NuGetAudit=true - Enable scanning
  • NuGetAuditLevel=low - Report all severity levels
  • NuGetAuditMode=all - Audit direct and transitive dependencies

Current package set has no known vulnerabilities.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[release/8.0.4xx] Enable Nuget Audit</issue_title>
<issue_description>Disabled in https://dev.azure.com/dnceng/internal/_git/dotnet-sdk/pullrequest/54678.
We should re enable it and bump some packages</issue_description>

<agent_instructions>It was disabled in directory.build.props. Reenable it and see what it flags.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [release/8.0.4xx] Enable Nuget Audit #51466

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[marcpopMSFT]

@dkurepa was you're issue specifically about turning audit back on in 8 or is main sufficient?
#51466