Bump NuGet.Packaging to 7.9.0 to match the SDK's NuGet version

[dennisdoomen]

Fixes #638.

What changed

NuGet.Packaging 6.14.3 → 7.9.0 in Directory.Packages.props, plus two NoWarn entries for the NU1701 warning this creates. Also pins NuGet.Protocol and NuGet.Resolver to 7.9.0 (see below). No target framework changes, no public API changes.

Why

The .NET 10 SDK loads NuGet.Frameworks 7.9.0.0 into the MSBuild process. The pinned NuGet.Packaging 6.14.3 loads NuGet.Frameworks 6.14.3 into that same process, so every ./build.sh run fails at Compile on SDK 10.0.400 with:

InvalidProjectFileException: The expression "[MSBuild]::GetTargetFrameworkIdentifier(net10.0)" cannot be evaluated. Could not load file or assembly 'NuGet.Frameworks, Version=7.9.0.0'.

NuGet 7.0 only removed APIs that were already marked obsolete. This repo uses NuspecReader, PackageArchiveReader, PackageFolderReader, VersionRange, NuGetVersion, and NuGetFramework. None of them changed.

NuGet 7.x no longer ships a netstandard2.0 build. Fallout.Tooling and Fallout.SourceGenerators still target netstandard2.0, so restoring them now pulls in .NET Framework compatibility assets instead and emits NU1701. That is a warning, not an error. Both NoWarn additions in this PR just silence it. No target framework is removed and no consumer-facing behavior changes.

The NuGet.Protocol / NuGet.Resolver pin

The first version of this PR broke all 14 tests in Fallout.Migrate.Analyzers.Specs with System.MethodAccessException on NuGet.Frameworks.NuGetFrameworkFullComparer..ctor().

Those specs use Microsoft.CodeAnalysis.CSharp.Analyzer.Testing.XUnit 1.1.2, which pins its Microsoft.CodeAnalysis.Analyzer.Testing dependency to exactly 1.1.2. That package in turn pins NuGet.Protocol and NuGet.Resolver to 6.3.4. Bumping NuGet.Packaging to 7.9.0 moved every package below it (NuGet.Common, NuGet.Configuration, NuGet.Frameworks, NuGet.Versioning) up to 7.9.0 through central transitive pinning, since they are dependencies of NuGet.Packaging. NuGet.Protocol and NuGet.Resolver depend on NuGet.Packaging instead of the other way round, so transitive pinning does not touch them, and they stayed at 6.3.4. Microsoft.CodeAnalysis.Testing's ReferenceAssemblies helper resolves packages over NuGet.Protocol at test time, so it now called into a NuGet.Frameworks build whose member accessibility no longer matched what the 6.3.4 build of NuGet.Protocol expected. That is the source of the MethodAccessException.

There is no newer version of Microsoft.CodeAnalysis.CSharp.Analyzer.Testing.XUnit to pick up a matching NuGet.Protocol version. Its nuspec pins Microsoft.CodeAnalysis.Analyzer.Testing to the exact range [1.1.2, 1.1.2], so a central override cannot bump the base package either. Pinning NuGet.Protocol and NuGet.Resolver to 7.9.0 directly in Directory.Packages.props is the only way to unify the whole NuGet.* family, and CentralPackageTransitivePinningEnabled picks up both pins with no other project changes needed.

Relation to #639

#639 carries the same version bump but treats it as a breaking change. It uses target/vNext instead of target/vCurrent and holds the fix for next year's major, reasoning that netstandard2.0 support is going away. This PR keeps every netstandard2.0 target as-is. The warning is cosmetic, so this can ship now as an ordinary bug fix and close #638 immediately instead of waiting for the next major.

Verification

• ./build.sh Compile fails on SDK 10.0.400 without this change, succeeds with it.
• dotnet test tests/Fallout.Migrate.Analyzers.Specs : 14/14 pass (all 14 failed before the NuGet.Protocol/NuGet.Resolver pin was added).
• dotnet restore fallout.slnx: clean, no new warnings.

I've also verified this end-to-end against a real consumer:

1. Created a local version of the Fallout NuGet packages with the changes in this PR.

2. First reproduced the issue in FluentAssertions by removing the <PackageReference Include="NuGet.Frameworks" Version="7.9.0" /> on .NET SDK 10.0.400. Note that I had to downgrade from 10.0.401 to 10.0.400 to reproduce it. This gave me

   InvalidProjectFileException: The expression "[MSBuild]::GetTargetFrameworkIdentifier(net8.0)" cannot be evaluated. Could not load file or assembly 'NuGet.Frameworks, Version=7.9.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'. The located assembly's manifest definition does not match the assembly reference. (0x80131040) C:\Program Files\dotnet\sdk\10.0.401\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.TargetFrameworkInference.targets

3. Bumped the Fallout.Common and Fallout.Components references to the locally built pre-release Fallout packages.

4. Verified that the error went away.

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