[DeviceProvisioningService] Fix #12154: Enrollment cmdlet does not assign linked IoT Hubs when chosisng Custom allocation policy

[anusapan]

Description

Fixing the issue of below cmdlets that do not assign and retain linked IotHubs whenever choosing Custom allocation policy

• Set-AzIoTDeviceProvisioningServiceEnrollmentGroup

• Set-AzIoTDeviceProvisioningServiceEnrollment

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate ChangeLog.md file(s) has been updated:
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• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • a cmdlet design review was approved for the changes in this repository (Microsoft internal only)
  • the markdown help files have been regenerated using the commands listed here