Add validation for fallback_behavior
What does this MR do and why?
Add validation for fallback_behavior
- disable rule mode for invalid values
Changelog: changed
EE: true
MR acceptance checklist
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Screenshots or screen recordings
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How to set up and validate locally
Numbered steps to set up and validate the change are strongly suggested.
- Upload a GitLab Ultimate license
- Navigate to a project => Secure => Policies => New policy => Merge request approval policy (example)
- Navigate to yaml, change fallback_behavior to something invalid
Related to #451662 (closed)