Notif Dispatch ImmutableConfig ParseError in Drupal 9.3.5

Problem/Motivation

Installing the 1.x-dev release of the Notification Systems Dispatch Module gives this error:
The website encountered an unexpected error. Please try again later.
ParseError: syntax error, unexpected 'ImmutableConfig' (T_STRING), expecting function (T_FUNCTION) or const (T_CONST) in Composer\Autoload\includeFile() (line 49 of modules/notification_system/modules/notification_system_dispatch/src/Service/NotificationDispatcherService.php).

Steps to reproduce

Download the 1.-x-dev release and install the core module along with the dispatch module.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes