Exclude Go 1.18 from CI

.github/workflows/test.yml

@@ -14,7 +14,7 @@ jobs:
       image: darthsim/imgproxy-ci:202302212110
     strategy:
       matrix:
-        go-version: ["1.20.x", "1.19.x", "1.18.x"]
+        go-version: ["1.20.x", "1.19.x"]
         vips-version: ["8.14", "8.13"]
     steps:
       - name: Checkout