Add Go 1.20 to tested Go versions (#303)

Author: jihuin

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        go-version: [ '1.11.x', '1.12.x', '1.13.x', '1.14.x', '1.15.x', '1.16.x', '1.18.x', '1.19.x']
+        go-version: [ '1.11.x', '1.12.x', '1.13.x', '1.14.x', '1.15.x', '1.16.x', '1.18.x', '1.19.x', '1.20.x']
 
     steps:
     - name: Set up Go