Update cimg/go Docker tag to v1.21.4

Author: renovate[bot]

.circleci/config.yml

@@ -8,7 +8,7 @@ jobs:
   test_go:
     resource_class: medium
     docker:
-      - image: cimg/go:1.19.1
+      - image: cimg/go:1.21.4
     steps:
       - checkout
       - attach_workspace:
@@ -23,7 +23,7 @@ jobs:
   build_go:
     resource_class: large
     docker:
-      - image: cimg/go:1.19.1
+      - image: cimg/go:1.21.4
     steps:
       - checkout
       - attach_workspace:
@@ -48,7 +48,7 @@ jobs:
   publish_gh:
     resource_class: small
     docker:
-      - image: cimg/go:1.19.1
+      - image: cimg/go:1.21.4
     steps:
       - attach_workspace:
           at: .