ci: bump actions/checkout from 5 to 6 (#63)

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Author: dependabot[bot]

.github/workflows/upstream_formula_update.yml

@@ -19,7 +19,7 @@ jobs:
       - uses: hmarr/debug-action@v3.0.0
 
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           token: ${{ secrets.GIT_PAT }}
           ref: ${{ github.event.repository.default_branch }}