Deprecate dynamic milestone pages

• In GitLab currently we have support for project milestones and group milestones.
• For a given issue or merge request, you can assign a project milestone to it, from it's project.
• For a given issue or merge request, you can assign a group milestone to it, from it's parent group.
  • In the future, you should be able to assign a group milestone from further ancestor groups, i.e. https://gitlab.com/gitlab-org/gitlab-ce/issues/36862 and &22 (closed) more broadly.
• This scheme is straightforward and simple to understand. And it is already very powerful/flexible for users.

Currently, GitLab still has some legacy features we want to remove:

• For a given instance, there is a page that pulls in all the project milestones with the same name, in the entire instance. You can navigate to this dynamic page by going to the instance milestones dashboard (e.g. https://gitlab.com/dashboard/milestones) and clicking any of the "project milestones" there. It will send you to a dynamic page (e.g. https://gitlab.com/dashboard/milestones/95?title=9.5).
• For a given group, there is a page that pulls in all the project milestones with the same name, in that group. You can navigate to this dynamic page by going to the Group Milestones page (e.g. https://gitlab.com/groups/gitlab-org/-/milestones) and clicking any of the "project milestones" there. It will send you to a dynamic page (e.g. https://gitlab.com/groups/gitlab-org/-/milestones/100?title=10.0).

To be done before this issue

• https://gitlab.com/gitlab-org/gitlab-ce/issues/42336 Put a deprecation warning on the dynamic milestone page at least one release prior to removing it.

Scope for this issue

• Remove the dynamic milestone pages, i.e. the dynamically generated pages themselves.
• Remove the links to these dynamic milestone pages. As linked to above, in the dashboard milestone list view, and in the group milestone list view, you can access these dynamically generated pages. These links should be removed.
• For the group milestone list view, we should retain listing of project milestones because of #24779 (closed) but these should now link to the project milestone page