Remove obsolete/unnecessary NuGet dependencies and bump version of remaining ones

[DoctorKrolic]

Closes https://github.com/tintoy/msbuild-project-tools-server/pull/97 Closes https://github.com/tintoy/msbuild-project-tools-server/pull/96 Closes https://github.com/tintoy/msbuild-project-tools-server/pull/95

• Bumped version of primary NuGet dependencies to 6.0.6 to adress dependabot alerts
• It turned out NuGet.PackageManagement includes all other packages as transitive dependencies, so we don't need to explicitly list them
• Removed NuGet.Client dependency since we don't use it and the package itself is considered obsolete