Update actions/upload-artifact@v5 -> v6

Author: dtolnay

.github/workflows/ci.yml

@@ -35,7 +35,7 @@ jobs:
         run: echo RUSTFLAGS=${RUSTFLAGS}\ -Zrandomize-layout >> $GITHUB_ENV
         if: matrix.rust == 'nightly'
       - run: cargo test
-      - uses: actions/upload-artifact@v5
+      - uses: actions/upload-artifact@v6
         if: matrix.rust == 'nightly' && always()
         with:
           name: Cargo.lock