Remove k8s 1.18 from PR system tests

Author: ChunyiLyu

.github/workflows/pr.yml

@@ -52,7 +52,6 @@ jobs:
     strategy:
       matrix:
         k8s:
-        - v1.18.19
         - v1.19.11
         - v1.20.7
         - v1.21.1