Mention that kubernetes variables are deprecated

doc/ci/variables/predefined_variables.md

@@ -14,7 +14,7 @@ Some variables are only available with more recent versions of [GitLab Runner](h
 You can [output the values of all variables available for a job](index.md#list-all-environment-variables)
 with a `script` command.
 
-There are also [Kubernetes-specific deployment variables](../../user/project/clusters/deploy_to_cluster.md#deployment-variables).
+There are also [Kubernetes-specific deployment variables (deprecated)](../../user/project/clusters/deploy_to_cluster.md#deployment-variables).
 
 | Variable                                 | GitLab | Runner | Description |
 |------------------------------------------|--------|--------|-------------|