Merge pull request #7029 from rouault/backport_fix_perms

backport.yml: try to fix permissions

.github/workflows/backport.yml

@@ -5,12 +5,17 @@ on:
       - closed
       - labeled
 
-permissions: {}
+permissions:
+  contents: read
 
 jobs:
   backport:
     runs-on: ubuntu-latest
     name: Backport
+
+    permissions:
+      pull-requests: write
+
     steps:
       - name: Backport Bot
         id: backport