Backport Admin role fix to 15.10

What does this MR do and why?

In 15.10 !111904 (merged) introduced support for external groups and recently a bug was surfaced from that feature that de-priviliged admins into regular users. The appropriate fix has been merged into master for 15.11 however it's high severity and requires to be be patched onto 15.10.

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 original MR has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• Ensure the e2e:package-and-test job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

The process of backporting bug fixes into stable branches is tracked as part of an internal pilot. If you have questions about this process, please:

• Refer to the internal pilot issue for feedback or questions.
• Refer to the patch release runbook for engineers and maintainers for guidance.