PLT-6554 Add config setting to control enabling API version 3 (#6835)

* Add config setting to control enabling API version 3

* Update help text for APIv3 config setting (#6843)

* Update configuration_settings.jsx

* Update en.json

i18n/en.json

@@ -895,6 +895,10 @@
     "id": "api.context.404.app_error",
     "translation": "Sorry, we could not find the page."
   },
+  {
+    "id": "api.context.v3_disabled.app_error",
+    "translation": "API version 3 has been disabled on this server. Please use API version 4. See https://api.mattermost.com for details."
+  },
   {
     "id": "api.context.invalid_body_param.app_error",
     "translation": "Invalid or missing {{.Name}} in request body"