Add deprecated message to old approval files

**Why?**
A recent conitrbutor spent time updating these files
because it wasn't clear that they will be removed.

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10364

ee/app/assets/javascripts/vue_merge_request_widget/components/approvals/single_rule/approvals.vue

 <script>
+/* === WARNING ===
+ * This file will be removed pending the removal of the `approval_rules` feature flag.
+ *
+ * If a new feature needs to be added, please please make changes in the `./multiple_rule`
+ * directory (see the feature issue https://gitlab.com/gitlab-org/gitlab-ee/issues/1979).
+ *
+ * Follow along via this issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/10685.
+ */
+
 import createFlash from '~/flash';
 import MrWidgetContainer from '~/vue_merge_request_widget/components/mr_widget_container.vue';
 import MrWidgetIcon from '~/vue_merge_request_widget/components/mr_widget_icon.vue';

ee/app/assets/javascripts/vue_merge_request_widget/components/approvals/single_rule/approvals_body.vue

 <script>
+/* === WARNING ===
+ * This file will be removed pending the removal of the `approval_rules` feature flag.
+ *
+ * If a new feature needs to be added, please please make changes in the `./multiple_rule`
+ * directory (see the feature issue https://gitlab.com/gitlab-org/gitlab-ee/issues/1979).
+ *
+ * Follow along via this issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/10685.
+ */
+
 import { n__, s__, sprintf } from '~/locale';
 import Flash from '~/flash';
 import Icon from '~/vue_shared/components/icon.vue';

ee/app/assets/javascripts/vue_merge_request_widget/components/approvals/single_rule/approvals_footer.vue

 <script>
+/* === WARNING ===
+ * This file will be removed pending the removal of the `approval_rules` feature flag.
+ *
+ * If a new feature needs to be added, please please make changes in the `./multiple_rule`
+ * directory (see the feature issue https://gitlab.com/gitlab-org/gitlab-ee/issues/1979).
+ *
+ * Follow along via this issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/10685.
+ */
+
 import Flash from '~/flash';
 import Icon from '~/vue_shared/components/icon.vue';
 import UserAvatarLink from '~/vue_shared/components/user_avatar/user_avatar_link.vue';