Validate reserved report_approvers rule names through schema

What does this MR do and why?

Include reserved report_approvers rule names as part of json schema validation. This is part of a three-part solution for #370808 (closed).

Screenshots or screen recordings

How to set up and validate locally

1. Navigate to Security & Compliance > Policies, click New Policy
2. Create a new Scan result policy
3. Input either License-Check or Coverage-Check into name
4. Validation should fail instead of allow a new MR to be created

MR acceptance checklist

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