[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/templates.go

@@ -181,6 +181,9 @@ func GetChannelTemplate(cfg *model.Config) *putindextemplate.Request {
 			"type": types.KeywordProperty{
 				Type: "keyword",
 			},
+			"delete_at": types.LongNumberProperty{
+				Type: "long",
+			},
 		},
 	}