Assign group milestone from ancestor groups to issue or merge request

Description

• Given I am in an issue or merge request, assign any group milestone that is in an ancestor group (not just the parent group).
• Sidebar assignment in an issue or merge request.
• Sidebar assignment in issue board card.
• Quick actions.
• API should be updated. (https://gitlab.com/gitlab-org/gitlab-ce/issues/36862#note_76272967)
• Update the group burndown chart scope so that a group burndown chart considers all issues nesting down in all subgroups' projects. I.e. we weren't able to do it originally (https://gitlab.com/gitlab-org/gitlab-ee/issues/3064#note_68150464), but now we should update it that feature to support it. (https://gitlab.com/gitlab-org/gitlab-ce/issues/36862#note_76272967)

Not in scope

• https://gitlab.com/gitlab-org/gitlab-ce/issues/30106 is viewing these issues in the group list. That is not in scope.
• https://gitlab.com/gitlab-org/gitlab-ee/issues/3342 is viewing these issues in a group board. That is not in scope.

Original Description

Description

Current problem for me is that I have a lot of microservies all in their own repo and some of them in subgroups. I would now like to do a code iteration and track it with a milestone. Which works for every project that's not in a subgroup.

Proposal

As it might be unwanted by some people I would be fine with having it as opt-in on a group milestone.

Use cases

Pretty much every team that is working on a iteration together but has multiple projects in their group.