Remove the async-io pin from CI.

Author: sunfishcode

.github/workflows/main.yml

@@ -375,7 +375,6 @@ jobs:
     - name: Use specific dependency versions for Rust 1.63 compatibility.
       run: |
         cargo update --package=once_cell --precise=1.20.3
-        cargo update --package=async-io --precise=2.5.0
         cargo update --package=polling --precise=3.10.0
 
     # Don't use --all-features because some of the features have dependencies