Update actions/upload-artifact@v5 -> v6

Author: dtolnay

.github/workflows/ci.yml

@@ -30,7 +30,7 @@ jobs:
       - run: cargo check --no-default-features --features alloc
       - run: cargo test
         if: matrix.rust != '1.68.0'
-      - uses: actions/upload-artifact@v5
+      - uses: actions/upload-artifact@v6
         if: matrix.rust == 'nightly' && always()
         with:
           name: Cargo.lock