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

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

* Fix test.

config/default.json

@@ -101,7 +101,7 @@
         "MaxNotificationsPerChannel": 1000,
         "EnableConfirmNotificationsToChannel": true,
         "TeammateNameDisplay": "username",
-        "ViewArchivedChannels": true,
+        "ExperimentalViewArchivedChannels": false,
         "ExperimentalEnableAutomaticReplies": false,
         "ExperimentalHideTownSquareinLHS": false,
         "ExperimentalTownSquareIsReadOnly": false,