Create mechanism to restrict non-course instructors being selected as PR reviewers

When requesting a reviewer for a PR, [students are requested in the course instructions to select their specific instructor as a reviewer to their PR](https://github.com/CUAHSI/learning-gitflows-template/blob/6ce68bb5b35b22769884dfa1e159761486356962/course_guide_learners_static.md?plain=1#L181). We had a case where a student selected both the instructor and other members in the [CUAHSI-Gitflows-learners](https://github.com/CUAHSI-GitFlows-Learners) repository as reviewers. It would be good to have the capacity to have a mechanism that only allows the course instructor to be selected as a reviewer for future student trainings.

There was a solution explored through [creating a CODEOWNERS file in a test repo](726cfeccea10dff72d77a9f407d54c9447191d5b) and enabling branch protection to require review from the code owner(s):


This seemed to still not prevent the tagging of non-code owner individuals so we will want to look into a modified solution in the future.