Swagger: Rename annotations model (#77605)

Currently, in the schema, it's a global definition called `ItemDTO`. It's hard to figure out what that is
Renaming it to `Annotation` should be more helpful

pkg/services/annotations/models.go

@@ -87,6 +87,7 @@ func (i Item) TableName() string {
 	return "annotation"
 }
 
+// swagger:model Annotation
 type ItemDTO struct {
 	ID           int64            `json:"id" xorm:"id"`
 	AlertID      int64            `json:"alertId" xorm:"alert_id"`