CI: automatically add a warning comment to PRs with backport labels (#34132)

* CI: automatically add a warning comment to PRs with backport labels

* Update .mergify.yml

Co-authored-by: Trent Nelson <trent.a.b.nelson@gmail.com>

* Update .mergify.yml

Co-authored-by: Trent Nelson <trent.a.b.nelson@gmail.com>

---------

Co-authored-by: Trent Nelson <trent.a.b.nelson@gmail.com>

.mergify.yml

@@ -98,6 +98,19 @@ pull_request_rules:
         ignore_conflicts: true
         branches:
           - v1.16
+  - name: v1.16 backport warning comment
+    conditions:
+      - label=v1.16
+    actions:
+      comment:
+        message: >
+          Backports to the stable branch are to be avoided unless absolutely
+          necessary for fixing bugs, security issues, and perf regressions.
+          Changes intended for backport should be structured such that a
+          minimum effective diff can be committed separately from any
+          refactoring, plumbing, cleanup, etc that are not strictly
+          necessary to achieve the goal. Any of the latter should go only
+          into master and ride the normal stabilization schedule.
   - name: v1.17 feature-gate backport
     conditions:
       - label=v1.17
@@ -122,6 +135,21 @@ pull_request_rules:
         ignore_conflicts: true
         branches:
           - v1.17
+  - name: v1.17 backport warning comment
+    conditions:
+      - label=v1.17
+    actions:
+      comment:
+        message: >
+          Backports to the beta branch are to be avoided unless absolutely
+          necessary for fixing bugs, security issues, and perf regressions.
+          Changes intended for backport should be structured such that a
+          minimum effective diff can be committed separately from any
+          refactoring, plumbing, cleanup, etc that are not strictly
+          necessary to achieve the goal. Any of the latter should go only
+          into master and ride the normal stabilization schedule. Exceptions
+          include CI/metrics changes, CLI improvements and documentation
+          updates on a case by case basis.
 
 commands_restrictions:
   # The author of copied PRs is the Mergify user.