Update actions/upload-artifact@v5 -> v6

Author: dtolnay

.github/workflows/ci.yml

@@ -40,7 +40,7 @@ jobs:
  - run: cargo test --features small
  - run: cargo build --tests --features no-panic --release
  if: matrix.rust == 'nightly'
- - uses: actions/upload-artifact@v5
+ - uses: actions/upload-artifact@v6
  if: matrix.rust == 'nightly' && always()
  with:
  name: Cargo.lock