MM-8593 Default APIv3 to disabled for new installs (#8353)

* Default APIv3 to disabled for new installs

* Add EnableAPIv3 setting to client config

* Enable APIv3 for needed tests

config/default.json

@@ -22,7 +22,7 @@
         "EnableOnlyAdminIntegrations": true,
         "EnablePostUsernameOverride": false,
         "EnablePostIconOverride": false,
-        "EnableAPIv3": true,
+        "EnableAPIv3": false,
         "EnableLinkPreviews": false,
         "EnableTesting": false,
         "EnableDeveloper": false,