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>

models/migrations/fixtures/Test_updateOpenMilestoneCounts/expected_milestone.yml

@@ -0,0 +1,19 @@
+# type Milestone struct {
+#   ID              int64                  `xorm:"pk autoincr"`
+#   IsClosed        bool
+#   NumIssues       int
+#   NumClosedIssues int
+#   Completeness    int  // Percentage(1-100).
+# }
+-
+  id: 1
+  is_closed: false
+  num_issues: 3
+  num_closed_issues: 1
+  completeness: 33
+-
+  id: 2
+  is_closed: true
+  num_issues: 5
+  num_closed_issues: 5
+  completeness: 100

models/migrations/fixtures/Test_updateOpenMilestoneCounts/issue.yml

@@ -0,0 +1,25 @@
+# type Issue struct {
+#   ID               int64 `xorm:"pk autoincr"`
+#   RepoID           int64 `xorm:"INDEX UNIQUE(repo_index)"`
+#   Index            int64 `xorm:"UNIQUE(repo_index)"` // Index in one repository.
+#   MilestoneID      int64 `xorm:"INDEX"`
+#   IsClosed         bool  `xorm:"INDEX"`
+# }
+-
+  id: 1
+  repo_id: 1
+  index: 1
+  milestone_id: 1
+  is_closed: false
+-
+  id: 2
+  repo_id: 1
+  index: 2
+  milestone_id: 1
+  is_closed: true
+-
+  id: 4
+  repo_id: 1
+  index: 3
+  milestone_id: 1
+  is_closed: false

models/migrations/fixtures/Test_updateOpenMilestoneCounts/milestone.yml

@@ -0,0 +1,19 @@
+# type Milestone struct {
+#   ID              int64                  `xorm:"pk autoincr"`
+#   IsClosed        bool
+#   NumIssues       int
+#   NumClosedIssues int
+#   Completeness    int  // Percentage(1-100).
+# }
+-
+  id: 1
+  is_closed: false
+  num_issues: 4
+  num_closed_issues: 2
+  completeness: 50
+-
+  id: 2
+  is_closed: true
+  num_issues: 5
+  num_closed_issues: 5
+  completeness: 100