Update actions/upload-artifact@v5 -> v6

Author: dtolnay

.github/workflows/ci.yml

@@ -77,7 +77,7 @@ jobs:
         run: cargo test
         env:
           RUSTFLAGS: -Z allow-features= --cfg procmacro2_backtrace ${{env.RUSTFLAGS}}
-      - uses: actions/upload-artifact@v5
+      - uses: actions/upload-artifact@v6
         if: always()
         with:
           name: Cargo.lock