Update golang Docker tag to v1.21

This MR contains the following updates:

Package	Type	Update	Change
golang	image	minor	1.20-alpine -> 1.21-alpine

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.gitlab-ci.yml

@@ -27,7 +27,7 @@ lint:
 
 coverage:
   stage: test
-  image: golang:1.20-alpine
+  image: golang:1.23-alpine
   coverage: '/\(statements\)(?:\s+)?(\d+(?:\.\d+)?%)/'
   script:
     - go install gotest.tools/gotestsum@latest