Cannot create alert from "Add content"

Problem/Motivation

It is possible to create alerts by visiting `/admin/config/system/bootstrap-site-alert`. However, users are used to creating them by going to Content->Add Content and selecting Bootstrap SIte Alert. SInce upgrading Drupal from 8 to 9, this results in a 404, and the following error appears in the logs: `Error: Call to undefined function bootstrap_site_alert_severity_options() in options_allowed_values() (line 89 of [...]/core/modules/options/options.module`

Steps to reproduce

In the GUI menu, select Content->Add Content->Bootstrap SIte Alert.

Proposed resolution

I expect the resolution may be that we need to change something on our site, rather than it being a bug in the module. I would be grateful for any advice as to what might have caused that function to become undefined

Remaining tasks

User interface changes

API changes

Data model changes