Update cimg/go Docker tag to v1.20

Author: renovate[bot]

.circleci/config.yml

@@ -20,7 +20,7 @@ jobs:
 
   backend:
     docker:
-      - image: cimg/go:1.18
+      - image: cimg/go:1.20
     steps:
       - checkout
       - attach_workspace:
@@ -44,7 +44,7 @@ jobs:
 
   publish-github-release:
     docker:
-      - image: cimg/go:1.18
+      - image: cimg/go:1.20
     steps:
       - attach_workspace:
           at: .