Update actions/setup-node action to v6

Author: renovate[bot]

.github/workflows/diffy.yml

@@ -36,7 +36,7 @@ jobs:
       #          ${{ env.CACHE_VERSION}}-${{ runner.os }}-base-${{ hashFiles('scripts/setup.sh') }}-${{ hashFiles('scripts.diffy.sh') }}
       - name: Use Node.js
         #      if: steps.base-diff.outputs.cache-hit != 'true'
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@v6
         with:
           node-version: 20
       - name: Install diff2html