Make Resource::Milestone#all_active_with_start_date public

What does this MR do and why?

This MR makes Gitlab::Triage::Resource::Milestone#all_active_with_start_date public because it's being called by https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/lib/versioned_milestone.rb#L22 using __send__, which is a bit of a no-no.

Related

• Call `all_active_with_start_date` directly (gitlab-org/quality/triage-ops!3591 - merged)