Deploy mr once take 2

What does this MR do?

Second attempt to work on #199256 (closed) (first one was !24591 (closed))

This time we break the first normal form to add a uniqueness index to prevent duplication.

This MR adds a new environment_id column to deployment_merge_requests table, with a foreign_key and a uniqueness index on [environment_id, merge_request_id].

The already existing Deployment#link_merge_requests(relation) code will take care of dropping duplicates

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team