Bumps cluster-applications minor version to 0.15.0

Main changes include:

- Adding FluentD support
- Adding Knative support

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

 apply:
   stage: deploy
-  image: "registry.gitlab.com/gitlab-org/cluster-integration/cluster-applications:v0.13.1"
+  image: "registry.gitlab.com/gitlab-org/cluster-integration/cluster-applications:v0.15.0"
   environment:
     name: production
   variables:
@@ -17,6 +17,8 @@ apply:
     ELASTIC_STACK_VALUES_FILE: $CI_PROJECT_DIR/.gitlab/managed-apps/elastic-stack/values.yaml
     VAULT_VALUES_FILE: $CI_PROJECT_DIR/.gitlab/managed-apps/vault/values.yaml
     CROSSPLANE_VALUES_FILE: $CI_PROJECT_DIR/.gitlab/managed-apps/crossplane/values.yaml
+    FLUENTD_VALUES_FILE: $CI_PROJECT_DIR/.gitlab/managed-apps/fluentd/values.yaml
+    KNATIVE_VALUES_FILE: $CI_PROJECT_DIR/.gitlab/managed-apps/knative/values.yaml
   script:
     - gitlab-managed-apps /usr/local/share/gitlab-managed-apps/helmfile.yaml
   only: