ci(mergify): upgrade configuration to current format (#1326)

* ci(mergify): upgrade configuration to current format

* Use mergify only for backporting

---------

Co-authored-by: Mergify <37929162+mergify[bot]@users.noreply.github.com>
Co-authored-by: Ivan Carvalho <[email protected]>

Author: mergify[bot]

.github/.mergify.yml

@@ -0,0 +1,8 @@
+pull_request_rules:
+  - name: backport
+    conditions:
+      - label=stable-backport-potential
+    actions:
+      backport:
+        branches:
+          - stable/0.15