ci: bump MSRV to 1.66.0

Author: tommilligan

.github/workflows/check.yml

@@ -70,7 +70,7 @@ jobs:
         rust:
           - stable
           - beta
-          - 1.64.0
+          - 1.66.0
         experimental:
           - false
         # Run a canary test on nightly that's allowed to fail