Fix Get-TargetResource for AADGroup resource

[ajkenah]

Pull Request (PR) description

Updates the Get-TargetResource function for AADGroup to return a Schema accurate object that can be processed with the LCM.

This Pull Request (PR) fixes the following issues

FIXES #6796

Task list

• Added an entry to the change log under the Unreleased section of the file CHANGELOG.md.
  Entry should say what was changed and how that affects users (if applicable), and
  reference the issue being resolved (if applicable).
• Resource parameter descriptions added/updated in the schema.mof.
• Resource documentation added/updated in README.md.
• Resource settings.json file contains all required permissions.
• Examples appropriately added/updated.
• Unit tests added/updated.
• New/changed code adheres to DSC Community Style Guidelines.