Drop support for 2.16 branch

Author: ggainey

template_config.yml

@@ -103,7 +103,6 @@ stalebot_days_until_stale: 90
 stalebot_limit_to_pulls: true
 supported_release_branches:
 - '2.15'
-- '2.16'
 - '2.19'
 - '2.20'
 - '2.22'