MM-11782: Make archived channels experimental and off-by-default. (#9281)

* MM-11782: Make archived channels experimental and off-by-default.

* Fix test.

app/channel.go

@@ -1498,7 +1498,7 @@ func (a *App) UpdateChannelLastViewedAt(channelIds []string, userId string) *mod
 }
 
 func (a *App) AutocompleteChannels(teamId string, term string) (*model.ChannelList, *model.AppError) {
-	includeDeleted := *a.Config().TeamSettings.ViewArchivedChannels
+	includeDeleted := *a.Config().TeamSettings.ExperimentalViewArchivedChannels
 
 	if result := <-a.Srv.Store.Channel().AutocompleteInTeam(teamId, term, includeDeleted); result.Err != nil {
 		return nil, result.Err
@@ -1508,7 +1508,7 @@ func (a *App) AutocompleteChannels(teamId string, term string) (*model.ChannelLi
 }
 
 func (a *App) SearchChannels(teamId string, term string) (*model.ChannelList, *model.AppError) {
-	includeDeleted := *a.Config().TeamSettings.ViewArchivedChannels
+	includeDeleted := *a.Config().TeamSettings.ExperimentalViewArchivedChannels
 
 	if result := <-a.Srv.Store.Channel().SearchInTeam(teamId, term, includeDeleted); result.Err != nil {
 		return nil, result.Err