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/v229_test.go

@@ -0,0 +1,46 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package migrations
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/issues"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_updateOpenMilestoneCounts(t *testing.T) {
+	type ExpectedMilestone issues.Milestone
+
+	// Prepare and load the testing database
+	x, deferable := prepareTestEnv(t, 0, new(issues.Milestone), new(ExpectedMilestone), new(issues.Issue))
+	defer deferable()
+	if x == nil || t.Failed() {
+		return
+	}
+
+	if err := updateOpenMilestoneCounts(x); err != nil {
+		assert.NoError(t, err)
+		return
+	}
+
+	expected := []ExpectedMilestone{}
+	if err := x.Table("expected_milestone").Asc("id").Find(&expected); !assert.NoError(t, err) {
+		return
+	}
+
+	got := []issues.Milestone{}
+	if err := x.Table("milestone").Asc("id").Find(&got); !assert.NoError(t, err) {
+		return
+	}
+
+	for i, e := range expected {
+		got := got[i]
+		assert.Equal(t, e.ID, got.ID)
+		assert.Equal(t, e.NumIssues, got.NumIssues)
+		assert.Equal(t, e.NumClosedIssues, got.NumClosedIssues)
+	}
+}