Update milestone counters when issue is deleted (#21459)

Update milestone counters when issue is deleted (#21459)

When actions besides "delete" are performed on issues, the milestone
counter is updated. However, since deleting issues goes through a
different code path, the associated milestone's count wasn't being
updated, resulting in inaccurate counts until another issue in the same
milestone had a non-delete action performed on it.

I verified this change fixes the inaccurate counts using a local docker
build.

Fixes #21254

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>