ci: Bump docker version to 28

We got an error due to mismatch in between docker and docker client
version. To unify this we moved to version 28 everywhere.

Signed-off-by: Paweł Poławski <[email protected]>

Author: elkoniu

.gitlab-ci.yml

@@ -47,7 +47,7 @@ variables:
 
   DOCKER_VERSION:
     description: "Docker version to use in jobs"
-    value: "27.3"
+    value: "28"
 
   MENDER_SERVER_TAG:
     description: "Mender Server Docker tag for running integration tests"