Add top level warnings to deprecated views

- See the [combined menu issue][1].
- The combined_menu feature flag will remove these
  files.
- The logic is being moved to a `top_nav_helper`
  ([see MR][2]).

[1]: https://gitlab.com/gitlab-org/gitlab/-/issues/301143
[2]: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/56587

app/views/layouts/nav/_dashboard.html.haml

--# WAIT! Before adding more items to the nav bar, please see
--# https://gitlab.com/gitlab-org/gitlab-foss/issues/49713 for more information.
+-# WARNING! This file is slated to be removed along with the `combined_menu`
+-# feature flag. The logic here will be migrated to an upcoming `top_nav_helper`.
+-# Please see [this MR][1] for more context.
+-# [1]: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/56587
 %ul.list-unstyled.navbar-sub-nav
   - if dashboard_nav_link?(:projects)
     = nav_link(path: ['root#index', 'projects#trending', 'projects#starred', 'dashboard/projects#index'], html_options: { id: 'nav-projects-dropdown', class: "home dropdown header-projects qa-projects-dropdown", data: { track_label: "projects_dropdown", track_event: "click_dropdown", track_experiment: "new_repo" } }) do

app/views/layouts/nav/_explore.html.haml

+-# WARNING! This file is slated to be removed along with the `combined_menu`
+-# feature flag. The logic here will be migrated to an upcoming `top_nav_helper`.
+-# Please see [this MR][1] for more context.
+-# [1]: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/56587
 %ul.list-unstyled.navbar-sub-nav
   - if explore_nav_link?(:projects)
     = nav_link(path: ['dashboard#show', 'root#show', 'projects#trending', 'projects#starred', 'projects#index'], html_options: {class: 'home'}) do

app/views/layouts/nav/groups_dropdown/_show.html.haml

+-# WARNING! This file is slated to be removed along with the `combined_menu`
+-# feature flag. The logic here will be migrated to an upcoming `top_nav_helper`.
+-# Please see [this MR][1] for more context.
+-# [1]: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/56587
 - group_meta = { id: @group.id, name: @group.name, namespace: @group.full_name, web_url: group_path(@group), avatar_url: @group.avatar_url } if @group&.persisted?
 .frequent-items-dropdown-container
   .frequent-items-dropdown-sidebar.qa-groups-dropdown-sidebar

app/views/layouts/nav/projects_dropdown/_show.html.haml

+-# WARNING! This file is slated to be removed along with the `combined_menu`
+-# feature flag. The logic here will be migrated to an upcoming `top_nav_helper`.
+-# Please see [this MR][1] for more context.
+-# [1]: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/56587
 - project_meta = { id: @project.id, name: @project.name, namespace: @project.full_name, web_url: project_path(@project), avatar_url: @project.avatar_url } if @project&.persisted?
 .frequent-items-dropdown-container
   .frequent-items-dropdown-sidebar.qa-projects-dropdown-sidebar

ee/app/views/dashboard/_nav_link_list.html.haml

+-# WARNING! This file is slated to be removed along with the `combined_menu`
+-# feature flag. The logic here will be migrated to an upcoming `top_nav_helper`.
+-# Please see [this MR][1] for more context.
+-# [1]: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/56587
 - if dashboard_nav_link?(:environments)
   = link_to operations_environments_path, class: 'dropdown-item', data: { qa_selector: 'environment_link' } do
     = _('Environments')

ee/app/views/layouts/nav/_geo_primary_node_url.html.haml

+-# WARNING! This file is slated to be removed along with the `combined_menu`
+-# feature flag. The logic here will be migrated to an upcoming `top_nav_helper`.
+-# Please see [this MR][1] for more context.
+-# [1]: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/56587
 - if Gitlab::Geo.secondary? && Gitlab::Geo.primary_node_configured?
   %li
     = link_to Gitlab::Geo.primary_node.url, title: 'Go to primary node', data: {toggle: 'tooltip', placement: 'bottom', container: 'body'} do