Add ROS 2 Jazzy backport mergify rule (#466)

Signed-off-by: Ryan Friedman <[email protected]>

Author: Ryanf55

.github/.mergify.yml

@@ -1,5 +1,14 @@
 ---
 pull_request_rules:
+  - name: backport to jazzy at reviewers discretion
+    conditions:
+      - base=rolling
+      - label=backport-jazzy
+    actions:
+      backport:
+        branches:
+          - jazzy
+
   - name: backport to iron at reviewers discretion
     conditions:
       - base=rolling