azurerm_management_group - fix issue with child subscription dissociation

[teowa]

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Description

This seems a regression caused by change in PR #26430, a wrong parser is used to parse the subscription ID.

PR Checklist

• I have followed the guidelines in our Contributing Documentation.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
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Changes to existing Resource / Data Source

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Testing

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TF_ACC=1 go test -v ./internal/services/managementgroup -parallel 5 -run TestAccManagementGroup_withSubscription -timeout 2h -ldflags="-X=github.com/hashicorp/terraform-provider-azurerm/version.ProviderVersion=acc"
=== RUN   TestAccManagementGroup_withSubscriptions
=== PAUSE TestAccManagementGroup_withSubscriptions
=== CONT  TestAccManagementGroup_withSubscriptions
--- PASS: TestAccManagementGroup_withSubscriptions (121.91s)
PASS
ok      github.com/hashicorp/terraform-provider-azurerm/internal/services/managementgroup       121.942s

Change Log

Below please provide what should go into the changelog (if anything) conforming to the Changelog Format documented here.

• azurerm_management_group - fix issue with child subscription dissociation [azurerm_management_group - fix issue with child subscription dissociation #28228]

This is a (please select all that apply):

• Bug Fix
• New Feature (ie adding a service, resource, or data source)
• Enhancement
• Breaking Change

Related Issue(s)

Fixes #28205

Note

If this PR changes meaningfully during the course of review please update the title and description as required.

[stephybun]

Thanks @teowa LGTM 👍