update mattermost server build to use go 1.16.6 (#17969)

Signed-off-by: Carlos Panato <ctadeu@gmail.com>

.gitlab-ci.yml

@@ -12,7 +12,7 @@ include:
 
 variables:
   BUILD: "yes"
-  IMAGE_BUILD_SERVER: $CI_REGISTRY/mattermost/ci/images/mattermost-build-server:20201119_golang-1.15.5
+  IMAGE_BUILD_SERVER: $CI_REGISTRY/mattermost/ci/images/mattermost-build-server:20210720_golang-1.16.6
   IMAGE_BUILD_DOCKER: $CI_REGISTRY/mattermost/ci/images/mattermost-build-docker:19.03.14-1
   IMAGE_DIND: $CI_REGISTRY/mattermost/ci/images/docker-dind:19.03.14-1