chore(project): improve slack message templates
chore(project): improve slack message templates

- creates a proper slack @ notification for the SEC message
- adds a hint to the support notification that the release manager
  may add extra announcements

related to CAMTEAM-280

camunda/camunda-bpm-platform,camunda/camunda-bpm-platform,camunda/camunda-bpm-platform,camunda/camunda-bpm-platform,camunda/camunda-bpm-platform,camunda/camunda-bpm-platform