Earlier branch: Send a Slack notification on the monthly_release_initial_rc:update_release_status job

gitlab-org/release-tools!4377 (diffs) introduced a new job on the initial_rc stage of the monthly release pipeline. The job switches the status on the release info from open to closed. When preparing the release candidate for 18.3, the job was successfully executed, but no notification was sent to Slack https://ops.gitlab.net/gitlab-org/release/tools/-/jobs/19808354

The purpose of this issue is to expand the update_release_status job to send a Slack notification about the job execution, similar to other jobs: