Update to reflect current status of notify.gov.au

Problem/Motivation

This module allows use of three notifications platforms; UK, Canada, and Australia.

The platform at https://notify.gov.au/ now returns a 404.

The version of that page from March 2022 states the service is discontinued and links to https://web.archive.org/web/20220303171944/https://www.dta.gov.au/our-pr..., which says

These platforms are now ready for others to take forward. The DTA will not be undertaking any further development for these platforms but we are now sharing to other government agencies the materials developed during the program through our GitHub open-source code repository.

Unless it is found that this was taken forward by some other element of Australian Government, we should remove this from documentation and as a selectable option in the module.

If a version still exists in Australian Government, we should update the module to use the new service.

Steps to reproduce

N/A

Proposed resolution

Either remove or replace the details of the Australian service, and update all documentation, including the project page on Drupal.org.

Remaining tasks

All the things.

User interface changes

Australian option potentially removed from configuration.

API changes

None.

Data model changes

None.