Implement /ready quick action

What does this MR do and why?

Adds a /ready quick action to 'turn off' draft status on an MR.

This allows use in ways where an idempotent version would be useful.

Relates to #365365 (closed)

Screenshots or screen recordings

How to set up and validate locally

On a merge request (new or open) you can use the quick action /ready to idempotently set the MR to Ready.

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.