Bump Go version to v1.20 (#24439)

.gitlab-ci.yml

@@ -12,7 +12,7 @@ include:
 
 variables:
   BUILD: "yes"
-  IMAGE_BUILD_SERVER: $CI_REGISTRY/mattermost/ci/images/mattermost-build-server:20230118_golang-1.19.5
+  IMAGE_BUILD_SERVER: $CI_REGISTRY/mattermost/ci/images/mattermost-build-server:20230904_golang-1.20.7
   IMAGE_BUILD_DOCKER: $CI_REGISTRY/mattermost/ci/devops-images/mattermost-build-docker:23.0.1-0
   IMAGE_DIND: $CI_REGISTRY/mattermost/ci/devops-images/docker-dind:23.0.1-0