Version bump to 6.0 for the break to back compatibility in Update-UnityPackageManagerConfig - #271

Per discussion with Josh, we should bump to 6.0 because breaking changes were made in the public Update-UnityPackageManagerConfig.

Specifically:

• ProjectManifestPath must now be a leaf node file, existing scripts passing a folder path should now use SearchPath
• Scripts that were using a SearchDepth without a SearchPath will error (previously this was not validated, would simply go unused if you passed a non-folder target and a depth)