[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/platform/services/searchengine/bleveengine/search.go

@@ -315,7 +315,7 @@ func (b *BleveEngine) IndexChannel(_ request.CTX, channel *model.Channel, userID
 	return nil
 }
 
-func (b *BleveEngine) SearchChannels(teamId, userID, term string, isGuest bool) ([]string, *model.AppError) {
+func (b *BleveEngine) SearchChannels(teamId, userID, term string, isGuest, _ bool) ([]string, *model.AppError) {
 	// This query essentially boils down to (if teamID is passed):
 	// match teamID == <>
 	// AND