Update release environment script logging to use key-value structure

During the release environment deployment, a script is executed to update the merge request with a specific label to indicate inclusion.

Recently gitlab-org/gitlab!203301 (diffs), logging was added to this script, but it is not using the key-value structure defined on https://docs.gitlab.com/development/logging/#how-to-use-json-logging

The purpose of this issue is to update the logging on that script to use key-values, e.g.

 @logger.info("Found merge requests for deployment", deployment_id: deployment_id, merge_request_count: deployment_mrs.count)

insetad of

@logger.info("Found #{deployment_mrs.count} merge requests for deployment #{deployment_id}.")