Add deprecation notice to Managed-Cluster-Applications.gitlab-ci.yml

Changelog: changed

lib/gitlab/ci/templates/Managed-Cluster-Applications.gitlab-ci.yml

+################################################################################
+# WARNING
+################################################################################
+#
+# This template is DEPRECATED and scheduled for removal in GitLab 15.0
+# See https://gitlab.com/gitlab-org/gitlab/-/issues/333610 for more context.
+#
+# To get started with a Cluster Management Project, we instead recommend
+# using the updated project template:
+#
+# - Documentation: https://docs.gitlab.com/ee/user/clusters/management_project_template.html
+# - Source code: https://gitlab.com/gitlab-org/project-templates/cluster-management/
+#
+################################################################################
+
 apply:
   stage: deploy
   image: "registry.gitlab.com/gitlab-org/cluster-integration/cluster-applications:v0.43.1"