Fix StartDateTime comparison for AAD Entitlement Access Package Assignment

[FabienTschanz]

Pull Request (PR) description

This PR fixes an issue when comparing the StartDateTime property of an AADEntitlementManagementAccessPackageAssignmentPolicy. Previously, it was exported from datetime without any standardized format. Now, it will be exported in the "o" format. Correspondingly, the schema type was updated from DATETIME to String to match the comparison independent of the operating system and language version.

This Pull Request (PR) fixes the following issues

None.

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.