Add .github/workflows/auto_tag_stable.yml

This workflow automatically updates and pushes the 'stable' tag
when commits are pushed to the 9.5 branch

.github/workflows/auto_tag_stable.yml

@@ -0,0 +1,27 @@
+name: Update Stable Tag
+
+on:
+  push:
+    branches:
+      - 9.5
+
+permissions:
+    contents: read
+
+jobs:
+  update-stable-tag:
+    runs-on: ubuntu-latest
+    if: github.repository == 'OSGeo/PROJ'
+    permissions:
+        contents: write
+    steps:
+
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Tag
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          git tag -f stable
+          git push -f origin stable