[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/interface.go

@@ -33,7 +33,7 @@ type SearchEngineInterface interface {
 	// IndexChannel indexes a given channel. The userIDs are only populated
 	// for private channels.
 	IndexChannel(rctx request.CTX, channel *model.Channel, userIDs, teamMemberIDs []string) *model.AppError
-	SearchChannels(teamId, userID, term string, isGuest bool) ([]string, *model.AppError)
+	SearchChannels(teamId, userID, term string, isGuest, includeDeleted bool) ([]string, *model.AppError)
 	DeleteChannel(channel *model.Channel) *model.AppError
 	IndexUser(rctx request.CTX, user *model.User, teamsIds, channelsIds []string) *model.AppError
 	SearchUsersInChannel(teamId, channelId string, restrictedToChannels []string, term string, options *model.UserSearchOptions) ([]string, []string, *model.AppError)