Onboard new backport-pr re-usable github workflow (geospatial) (#872)

- Replace old backport workflow with reusable workflow from opensearch-build
- Remove obsolete backport-related workflows

Signed-off-by: Peter Zhu <zhujiaxi@amazon.com>

Author: peterzhuamazon

.github/workflows/backport.yml

@@ -1,28 +1,12 @@
+---
 name: Backport
 on:
   pull_request_target:
-    types:
-      - closed
-      - labeled
+    types: [closed, labeled]
 
 jobs:
   backport:
-    runs-on: ubuntu-latest
-    permissions:
-      contents: write
-      pull-requests: write
-    name: Backport
-    steps:
-      - name: GitHub App token
-        id: github_app_token
-        uses: tibdex/github-app-token@1901dc7d52169e70c27a8da37aef0d423e2867a2 # v1.5.0
-        with:
-          app_id: ${{ secrets.APP_ID }}
-          private_key: ${{ secrets.APP_PRIVATE_KEY }}
-          installation_id: 22958780
-
-      - name: Backport
-        uses: VachaShah/backport@28c49d91ceec57d7c9f625f1031c1a4d637251f5 # v1.1.4
-        with:
-          github_token: ${{ steps.github_app_token.outputs.token }}
-          branch_name: backport/backport-${{ github.event.number }}
+    if: github.repository == 'opensearch-project/geospatial'
+    uses: opensearch-project/opensearch-build/.github/workflows/backport-pr.yml@main
+    secrets:
+      OPENSEARCH_CI_BOT_TOKEN: ${{ secrets.OPENSEARCH_CI_BOT_TOKEN }}