ci: add go1.18 to tests matrix

Author: wI2L

.github/workflows/ci.yml

@@ -19,6 +19,7 @@ jobs:
           - "1.15.x"
           - "1.16.x"
           - "1.17.x"
+          - "1.18.x"
         os:
           - ubuntu-latest
           - macos-latest