Update actions/upload-artifact@v5 -> v6

Author: dtolnay

.github/workflows/ci.yml

@@ -31,7 +31,7 @@ jobs:
         working-directory: _skeptic
       - run: cargo test
         working-directory: _skeptic
-      - uses: actions/upload-artifact@v5
+      - uses: actions/upload-artifact@v6
         if: always()
         with:
           name: Cargo.lock