change the backport label

By request in Matrix. If anyone else has an opinion, speak now or
forever hold your peace! ☺

The new label is
`Backport #NNN: title`
which with GitHub's prefix and suffix for email is
`[haskell/cabal] Backport #NNN: title (PR #MMM)`

Author: geekosaur

.github/mergify.yml

@@ -170,3 +170,8 @@ queue_rules:
     update_bot_account: Mikolaj
     merge_method: squash
     update_method: merge
+
+defaults:
+  actions:
+    backport:
+      title: "Backport #{{ number }}: {{ title }}"