github-actions: copy gh labels to the mergify backports (#4306) (#4308)

(cherry picked from commit 76507d6)

Co-authored-by: Victor Martinez <victormartinezrubio@gmail.com>

.github/workflows/mergify-labels-copier.yml

@@ -0,0 +1,23 @@
+name: mergify backport labels copier
+
+on:
+  pull_request:
+    types:
+      - opened
+
+permissions:
+  contents: read
+
+jobs:
+  mergify-backport-labels-copier:
+    runs-on: ubuntu-latest
+    if: startsWith(github.head_ref, 'mergify/bp/')
+    permissions:
+      # Add GH labels
+      pull-requests: write
+      # See https://github.com/cli/cli/issues/6274
+      repository-projects: read
+    steps:
+      - uses: elastic/oblt-actions/mergify/labels-copier@v1
+        with:
+          excluded-labels-regex: "^backport-*"