Reduce build matrix

.github/workflows/ci.yml

@@ -9,8 +9,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        go-version: [1.12.x, 1.13.x, 1.14.x]
-        kafka-version: [2.3.1, 2.4.1, 2.5.0]
+        go-version: [1.14.x]
+        kafka-version: [2.4.1, 2.5.0]
         platform: [ubuntu-latest]
 
     env: