Add Go v1.21 to CI

Go v1.21 was just released today (https://go.dev/doc/go1.21), so let's
make sure our builds work with it.

Author: stanhu

.gitlab-ci.yml

@@ -78,7 +78,7 @@ default:
 .go-matrix-job:
   parallel:
     matrix:
-      - GO_VERSION: ["golang-1.19", "golang-1.20"]
+      - GO_VERSION: ["golang-1.19", "golang-1.20", "golang-1.21"]
 
 ################################################################################
 # Prepare jobs