Draft: Create Members::DowngradedToGuestEvent event

What does this MR do and why?

Create Members::DowngradedToGuestEvent event

This event will be used to notify subscribers when a user is downgraded to guest. Then the subscriber can perform cleanup task async based on the result

Screenshots or screen recordings

n/a

How to set up and validate locally

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

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 #415301