Add warn action type

What does this MR do and why?

Add warn action type

• allow for warn action type in approval policies
• the warn action type is unlike the other two actions as it does not have a new action object, but instead will be comprised of similar action yaml as the default action yaml, except the required approvers is 0
• The warn action can only exist by itself, so if a warn action exists and the user adds a non-warn action, we should clear the warn action and its approvers. Similarly, if another action exists and the user adds a warn action, we should clear the other actions and their approvers
• update tests

References

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MR acceptance checklist

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Screenshots or screen recordings

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

Scenario	Before	After
Action selection
Warn action added	-

NOTE: See the above screenshots for the most up-to-date design; this video will only be updated if the behavior changes in the MR POC_small

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. Enable the feature flag echo "Feature.enable(:security_policy_approval_warn_mode)" | rails c
3. Navigate to a project/group => Secure => Policies => New policy => Merge request approval policy
4. Verify the user can add the warn action
5. Verify the warn action removes all other actions
6. Verify when a user adds a different action, the warn action is removed
7. Verify the user can navigate to/from yaml mode and the warn action continues to show

Related to #504940 (closed)