[AI assisted]: MM-62295: Search and index archived channels as well. (#29796)

We add a new field delete_at in the channels template.

This field is then searched in the SearchChannels function.

Also added tests to verify that archived channels are searched
properly, and also indexed correctly.

https://mattermost.atlassian.net/browse/MM-62295

```release-note
- Now archived channels are searchable with ES/OS if TeamSettings.ExperimentalViewArchivedChannels is enabled.
- If there are old channels which were archived before a bulk index was run, users would need to purge indexes, and do bulk index again. Because those old archived channels are removed from the index when a bulk index is run.
```


Co-authored-by: Mattermost Build <build@mattermost.com>

server/enterprise/elasticsearch/common/common.go

@@ -68,6 +68,7 @@ type ESFile struct {
 type ESChannel struct {
 	Id            string            `json:"id"`
 	Type          model.ChannelType `json:"type"`
+	DeleteAt      int64             `json:"delete_at"`
 	UserIDs       []string          `json:"user_ids"`
 	TeamId        string            `json:"team_id"`
 	TeamMemberIDs []string          `json:"team_member_ids"`
@@ -206,6 +207,7 @@ func ESChannelFromChannel(channel *model.Channel, userIDs, teamMemberIDs []strin
 	return &ESChannel{
 		Id:            channel.Id,
 		Type:          channel.Type,
+		DeleteAt:      channel.DeleteAt,
 		UserIDs:       userIDs,
 		TeamId:        channel.TeamId,
 		TeamMemberIDs: teamMemberIDs,