Switch to the new $GITHUB_OUTPUT mechanism from `set-output` in CI

[fadeev]

🛠️ Refactor suggestion

Deprecated set-output – switch to the new $GITHUB_OUTPUT mechanism

The command was removed in October 2023 and will eventually error out. Refactor to:

-          git diff --cached --exit-code || echo "::set-output name=changed::true"
+          if ! git diff --cached --quiet; then
+            echo "changed=true" >> "$GITHUB_OUTPUT"
+          fi