remove 8.16 as an active branch (#7623)

Author: ebeahan

.mergify.yml

@@ -407,7 +407,6 @@ pull_request_rules:
           - "8.x"
           - "8.18"
           - "8.17"
-          - "8.16"
         labels:
           - "backport"
         title: "[{{ destination_branch }}](backport #{{ number }}) {{ title }}"
@@ -440,7 +439,6 @@ pull_request_rules:
           - "9.0"
           - "8.18"
           - "8.17"
-          - "8.16"
           - "8.x"
           - "7.17"
         labels: