Update .mergify.yml (#2192)

Adds the 8.4 branch to mergify configuration file

Author: lcawl

.mergify.yml

@@ -15,6 +15,20 @@ pull_request_rules:
  git merge <remote-repo>/{{base}}
  git push <remote-repo> {{head}}
  ```
+ - name: backport patches to 8.4 branch
+ conditions:
+ - merged
+ - base=main
+ - label=backport-8.4
+ actions:
+ backport:
+ assignees:
+ - "{{ author }}"
+ branches:
+ - "8.4"
+ title: "[{{ destination_branch }}] {{ title }} (backport #{{ number }})"
+ labels:
+ - backport
  - name: backport patches to 8.3 branch
  conditions:
  - merged
@@ -120,4 +134,4 @@ pull_request_rules:
  actions:
  comment:
  message: |
- This pull request has not been merged yet. Could you please review and merge it @{{ assignee | join(', @') }}? 🙏
+ This pull request has not been merged yet. Could you please review and merge it @{{ assignee | join(', @') }}? 🙏