Add milestones to release blocks

What does this MR do?

Adds milestones to release blocks as described in https://gitlab.com/gitlab-org/gitlab-ce/issues/62402:

UX specs

With no milestones associated (no visual changes):

With multiple milestones associated:

Note: the footer ("View Issues of Merge Requests in this release") is not implemented by this merge request.

Screenshots

With no milestones associated (no visual changes):

With one milestone associated:

With multiple milestones associated:

When hovering over a milestone:

This MR has a CE backport

gitlab-org/gitlab-ce!32290