Add label area/native-image to issues mentioning mandrel

As mandrel is now the default builder image, most issues about native
contain mandrel in the body even if they are not mandrel specific.

Start labeling the issues as `area/native-image` and let the mandrel
team decide whether it's mandrel specific or not.

.github/quarkus-github-bot.yml

@@ -407,7 +407,7 @@ triage:
         - extensions/google-cloud-functions
         - integration-tests/google-cloud-functions
     - id: mandrel
-      labels: [area/mandrel]
+      labels: [area/native-image]
       titleBody: "mandrel"
       notify: [galderz, zakkak, Karm]
     - id: native-image