Backport #39958 by @SHIVANSHGARG07
The project milestone filter listed every milestone of the repo, or of
all accessible org repos for org projects, including ones with no issue
in the project. That made the dropdown noisy and unhelpful.
Both repo and org projects now list only milestones that have at least
one issue linked to the project, via a new `ProjectID` option on
`FindMilestoneOptions`.
Fixes: https://github.com/go-gitea/gitea/issues/39150
Co-authored-by: Shivansh Garg <shivanshgarg587@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>