Bump NuGet.Resolver from 6.6.1 to 6.8.0 in /Solutions

Bumps [NuGet.Resolver](https://github.com/NuGet/NuGet.Client) from 6.6.1 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.Resolver
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Solutions/Endjin.Adr.Cli/Endjin.Adr.Cli.csproj

@@ -48,7 +48,7 @@
     <PackageReference Include="NDepend.Path" Version="2.0.0" />
     <PackageReference Include="NuGet.Common" Version="6.6.1" />
     <PackageReference Include="NuGet.Packaging.Core" Version="6.6.1" />
-    <PackageReference Include="NuGet.Resolver" Version="6.6.1" />
+    <PackageReference Include="NuGet.Resolver" Version="6.8.0" />
     <PackageReference Include="Spectre.Console" Version="0.47.0" />
     <PackageReference Include="Spectre.Console.Cli" Version="0.47.0" />
     <PackageReference Include="YamlDotNet" Version="13.1.1" />