ci: run on go1.18 (#349)

Author: markus-wa

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ jobs:
   ci:
     strategy:
       matrix:
-        go-version: [1.11.x, 1.17.x]
+        go-version: [1.11.x, 1.18.x]
 
     runs-on: ubuntu-latest