MM-10649: soften Channels.ExtraUpdateAt deprecation (#8843)

The previous complete removal of this field resulted in an
incompatibility with 4.x servers that could not handle the now null
column field.

Instead, ensure this field is at least always set to 0, with a plan to
remove it altogether in a future release.

model/channel.go

@@ -44,6 +44,7 @@ type Channel struct {
 	Purpose       string `json:"purpose"`
 	LastPostAt    int64  `json:"last_post_at"`
 	TotalMsgCount int64  `json:"total_msg_count"`
+	ExtraUpdateAt int64  `json:"extra_update_at"`
 	CreatorId     string `json:"creator_id"`
 }
 
@@ -132,6 +133,7 @@ func (o *Channel) PreSave() {
 
 	o.CreateAt = GetMillis()
 	o.UpdateAt = o.CreateAt
+	o.ExtraUpdateAt = 0
 }
 
 func (o *Channel) PreUpdate() {