Generated issues should automatically be added to the milestone

When preparing the release notes, we automatically generate an issue for each merged release-note-worthy PR.

This should also add them to the milestone, but this work was cut from https://github.com/bevyengine/bevy-website/pull/1222 to reduce the complexity of the initial implementation. Follow-up to #1220.

Note that you must pass in the integer associated with the milestone, *not* the string name. This will likely involve fetching the list of milestones and attempting to match it.