When filtering merge requests by an environment name, the index used by
DeploymentMergeRequest.deployed_to() is not ideal. We can improve
performance by adding the following AND condition:

    AND environments.project_id = merge_requests.target_project_id

This condition forces the query to use the index on merge_requests
(project_id, name), instead of using the slower index (name
varchar_pattern_ops).

See https://gitlab.com/gitlab-org/gitlab/-/issues/270068 for more
information.