Update export of CIM instance to use common function in PlannerTask

[FabienTschanz]

Pull Request (PR) description

This PR updates the export of the PlannerTask resource to use the common Get-M365DSCDRGComplexTypeToString function to convert a complex type to its CIM equivalent instead of a custom function. Additionally, the permissions were updated to reflect the effective permissions required to access the resource.

This Pull Request (PR) fixes the following issues

• Fixes PlannerTask: Null-valued expression exception in Convert-M365DSCPlannerTaskAssignmentToCIMArray #6004

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.