chore: bump NuGet.PackageManagement from 6.7.0 to 6.8.0 in /src (#288)

Bumps [NuGet.PackageManagement](https://github.com/NuGet/NuGet.Client) from 6.7.0 to 6.8.0.
- [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]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

src/UnityNuGet/UnityNuGet.csproj

@@ -24,7 +24,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.Extensions.Options" Version="7.0.1" />
-    <PackageReference Include="NuGet.PackageManagement" Version="6.7.0" />
+    <PackageReference Include="NuGet.PackageManagement" Version="6.8.0" />
     <PackageReference Include="Microsoft.Extensions.Caching.Memory" Version="7.0.0" />
     <PackageReference Include="NUglify" Version="1.21.0" />
     <PackageReference Include="Scriban" Version="5.9.0" />