refactors audit notification to mail, adds test, adds alerts check to scheduler

[Godmartinz]

This adds an alerts_enabled check around the following scheduled daily alerts:

• inventory-alerts
• expiring-alerts
• expected-checkin
• upcoming-audits

This also refactors the Audit notification to use mailable.
A test has been added for this that checks

• due for audit
• overdue for audit
• not due yet

Fixes: #16221

[what-the-diff]

PR Summary

• Revamp of the Audit Report Notification Process:
  This PR primarily reconfigures how the system sends notifications about upcoming audits. Instead of the past notification system, the system will now utilize the SendUpcomingAuditMail to send out email notifications. This change not only simplifies the process by cutting out the AlertRecipient and accompanying notification use but also includes a check to verify if any assets are due for an audit before sending out an email.

• Refinement of Kernel Scheduling:
  The PR modifies the logic behind kernel scheduling to ensure that the audit update command only functions when alerts are activated in the app settings. This change makes sure not to waste system resources on unnecessary tasks.

• Introduction of New Mailable Class:
  The creation of the SendUpcomingAuditMail class is meant to handle the configuration and content of the email informed of upcoming audits. This new class helps maintain order and clarity for system-generated emails.

• Improvement to Feature Tests:
  The PR builds upon the ExpiringAlertsNotificationTest with a new evaluation for the SendUpcomingAuditMail. This test confirms that emails about upcoming and overdue auditable assets are sent correctly, thereby aiding in ensuring the accuracy and reliability of the system.

• Addition to Settings Helper:
  The inclusion of the setAuditWarningDays method in the Settings support class aids in altering audit warning durations. This addition makes the management of warning limits much more adaptable.