Add rule mode verification for severity levels

What does this MR do and why?

Add rule mode verification for severity levels

• implement check from yaml mode to rule mode to verify severity levels
• implement check for duplicates

Changelog: added

EE: true

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Upload a GitLab Ultimate license
2. Navigate to a project/group => Secure => Policies => New policy => Scan Result Policy
3. Verify rule mode is enabled
4. Add a Security Policy => Navigate to yaml mode => Add an invalid severity level => Navigate to rule mode
5. Verify rule mode is disabled
6. Navigate to yaml mode => replace the invalid severity level with a valid one => Navigate to rule mode
7. Verify rule mode is enabled

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.

Related to #419682 (closed)