chore: bump NuGet.PackageManagement from 6.8.0 to 6.9.1 in /src

Bumps [NuGet.PackageManagement](https://github.com/NuGet/NuGet.Client) from 6.8.0 to 6.9.1.
- [Release notes](https://github.com/NuGet/NuGet.Client/releases)
- [Commits](https://github.com/NuGet/NuGet.Client/commits)

---
updated-dependencies:
- dependency-name: NuGet.PackageManagement
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

src/Directory.Packages.props

@@ -10,7 +10,7 @@
     <PackageVersion Include="Microsoft.Extensions.Options.DataAnnotations" Version="8.0.0" />
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.9.0" />
     <PackageVersion Include="Microsoft.VisualStudio.Azure.Containers.Tools.Targets" Version="1.19.6" />
-    <PackageVersion Include="NuGet.PackageManagement" Version="6.8.0" />
+    <PackageVersion Include="NuGet.PackageManagement" Version="6.9.1" />
     <PackageVersion Include="Microsoft.Extensions.Caching.Memory" Version="8.0.0" />
     <PackageVersion Include="NUglify" Version="1.21.2" />
     <PackageVersion Include="nunit" Version="3.14.0" />