Backport of 'Allow subgroup-provisioned SAs to create subgroups' to 18.11

What does this MR do and why?

Backports !235396 (merged) ("Allow subgroup-provisioned SAs to create subgroups") to 18-11-stable-ee. The original fix removes the SubgroupProvisionedServiceAccountRestriction include from EE::Organizations::OrganizationPolicy, which was incorrectly causing subgroup-provisioned service accounts to receive 403 Forbidden on POST /api/v4/groups when creating subgroups under their provisioning namespace.

The intended security properties from !225090 (merged) are still enforced via GlobalPolicy (top-level group creation) and GroupPolicy / ProjectPolicy (service account creation).

Customer impact

NatWest (Signature, GitLab Dedicated, on 18.10.3-ee at time of report) was blocked from running their Terraform-based onboarding flow against subgroup-scoped service accounts. Telstra also flagged related friction with subgroup/project service accounts.

References

• Original MR: !235396 (merged)
• Original issue: #599158
• Breaking-change source: !225090 (merged)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.