Drop 0.18 from supported branches

Author: gerrod3

template_config.yml

@@ -105,7 +105,6 @@ stalebot_days_until_close: 30
 stalebot_days_until_stale: 90
 stalebot_limit_to_pulls: true
 supported_release_branches:
-  - "0.18"
   - "0.20"
   - "0.21"
   - "0.22"