New MR state: deployed
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### Description
We want to fully connect idea to production. I'm thinking of further ways we could do this.
It'd be very useful to be able to see whether a certain issue is deployed to production (or any other environment).
The original proposal was for issues also, but that will be covered in https://gitlab.com/groups/gitlab-org/-/epics/5099.
### Proposals
#### For MRs
- WHEN a MR is merged
- AND the commits are deployed to `production`
- THEN set the MR state to `deployed`
#### To consider
1. For EE: customise what it means to be deployed. E.g. set an environment name
1. Make the state name the name of the environment
1. Name: deployed / live / production
### Links / references
issue