Deprecate /draft toggle action via feature flag

What does this MR do and why?

Deprecates the /draft quick action's 'toggle' feature. Adds a feature flag to switch to the idempotent version.

Relates to #365365 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Enable the Feature flag: Feature.enable(:draft_quick_action_non_toggle)
2. Open a Merge request
3. Use /draft as a quick action - should not toggle, only set the MR as draft.

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.