Use bot access token for merge-up pull requests (#1255)

Author: alcaeus

.github/workflows/merge-up.yml

@@ -5,12 +5,8 @@ on:
     branches:
       - "v[0-9]+.[0-9]+"
 
-permissions:
-  contents: write
-  pull-requests: write
-
 env:
-  GH_TOKEN: ${{ github.token }}
+  GH_TOKEN: ${{ secrets.MERGE_UP_TOKEN }}
 
 jobs:
   merge-up:
@@ -24,6 +20,7 @@ jobs:
         with:
           # fetch-depth 0 is required to fetch all branches, not just the branch being built
           fetch-depth: 0
+          token: ${{ secrets.MERGE_UP_TOKEN }}
 
       - name: Create pull request
         id: create-pull-request