Clearly document end of life of this plugin for Moodle, and clarify totara support

[brendanheywood]

This plugin is now in Moodle core so we should:

• Update the version.php so this version of the plugin is end of life in Moodle 4.2
• update the readme
• Probably branch into a Totara specific stable as the only reason this should exist in say 4 years is as a totara only plugin
• Identify any factors not included within the core plugin that need to be retained (eg security questions) and decide what to do with them.