add backport label to prs (#8609)

Backport a PR by just adding the target branch's label, e.g. backport-to-v0.41.x.

Co-authored-by: Amaury <1293565+amaurym@users.noreply.github.com>
Co-authored-by: Alessio Treglia <alessio@tendermint.com>

.mergify.yml

@@ -8,3 +8,11 @@ pull_request_rules:
       merge:
         method: squash
         strict: true
+  - name: backport patches to v0.41.x branch
+    conditions:
+      - base=master
+      - label=backport/0.41.x (Stargate)
+    actions:
+      backport:
+        branches:
+          - release/v0.41.x