Add the ability to filter merge requests in UI by merged before/after date

This feature proposal was created on behalf of a GitLab Ultimate customer (internal link) who reported they are interested in this functionality in a support ticket (internal link)

Proposal

To add the ability to filter merge requests by merged before/after date when viewing merge requests for a project. This is currently possible in GraphQL

createdBefore: Time
Merge requests created before the timestamp.

deployedAfter: Time
Merge requests deployed after the timestamp.