Fix NU1701 release build failure for xunit.runner.visualstudio on netcoreapp3.1

[Copilot]

Description

xunit.runner.visualstudio 2.8.2 targets net452 and net6.0+ with no netcoreapp3.1 assets. NuGet falls back to .NETFramework and emits NU1701, which Directory.Build.props promotes to a fatal error in Release builds (TreatWarningsAsErrors=true), breaking dotnet build -c:release.

Fix: Added <NoWarn>NU1701</NoWarn> to the xunit.runner.visualstudio 2.8.2 PackageReference scoped to netcoreapp3.1 (and older TFMs):

<PackageReference Include="xunit.runner.visualstudio" Version="2.8.2" allowedVersions="[2.8.2]">
    <Version>[2.8.2]</Version>
    <NoWarn>NU1701</NoWarn>
    <PrivateAssets>all</PrivateAssets>
    <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
</PackageReference>

Type of change

• Bug fix

How Has This Been Tested?

• dotnet build tests/Wolfgang.Etl.Abstractions.Tests.Unit -c:release --framework netcoreapp3.1 — Build succeeded, 0 errors

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Screenshots (if applicable)

Additional context

The NoWarn suppression is scoped to the PackageReference itself (not project-wide), so it only silences this specific known-compatible fallback and does not mask other NuGet compatibility issues.

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