Incorrect configuration settings name

I encountered an error when trying to import the configuration for auditfiles to a different environment.

The import failed due for the following reasons:
Configuration auditfiles_config.settings
depends on the auditfiles_config
extension that will not be installed after import.

This is probably because the settings yaml file should be called "auditfiles.settings.yml" to match the module name.

The settings file name is derived from the config settings name used, so this should be changed everywhere.