Release notes for v1.15.0

Changelog since v1.14.0

Changes by Kind

Feature

• Expose Mattermost Operator metrics (#233, @Szymongib)
• Set Observed Generation status value (#238, @gabrieljackson)

Other (Cleanup or Flake)

• Bump default mattermost version to 5.37.1 (#239, @gabrieljackson)
• Cleanup Mattermost Operator RBAC role (#234, @Szymongib)