Optimize backport.yml

Use ENV var instead of empty job with output.
See hazelcast/hz-docs#1403

.github/workflows/backport.yml

@@ -3,32 +3,28 @@ on:
  push:
   branches:
    - main
+
+env:
+  MAINTENANCE_BRANCHES: "['5.8', '5.9', '5.10', '5.11', '5.12', '5.13', '5.14']"
+
 jobs:
-  get-maintenance-branches:
-    runs-on: ubuntu-latest
-    outputs:
-      branches: "['5.8', '5.9', '5.10', '5.11', '5.12', '5.13', '5.14']"
-    steps:
-      - run: exit 0
 
   backport-to-all-branch:
-    needs: get-maintenance-branches
     strategy:
       fail-fast: false
       matrix:
-        branch: ${{ fromJSON(needs.get-maintenance-branches.outputs.branches) }}
+        branch: ${{ fromJSON(env.MAINTENANCE_BRANCHES) }}
     uses: hazelcast/hz-docs/.github/workflows/backport-workflow.yml@main
     with:
       label-to-check-for: '["backport to all versions"]'
       target-branch: v/${{ matrix.branch }}
     secrets: inherit
 
   backport-to-specified-branch:
-    needs: get-maintenance-branches
     strategy:
       fail-fast: false
       matrix:
-        branch: ${{ fromJSON(needs.get-maintenance-branches.outputs.branches) }}
+        branch: ${{ fromJSON(env.MAINTENANCE_BRANCHES) }}
     uses: hazelcast/hz-docs/.github/workflows/backport-workflow.yml@main
     with:
       label-to-check-for: '["backport to ${{ matrix.branch }}"]'