pin itoa in CI for MSRV compatibility

Author: xtqqczze

.github/workflows/main.yml

@@ -66,6 +66,7 @@ jobs:
         cargo update --package=windows-sys@0.61.2 --precise=0.60.2
         cargo update --package=quote --precise=1.0.41
         cargo update --package=ryu --precise=1.0.20
+        cargo update --package=itoa --precise=1.0.15
 
     - run: >
         rustup target add