Migrated to new System.Assert class, added System namespace to several references

Core Unlocked Package Changes

This release doesn't have any functional changes (hopefully 😅) - it just contains some tech-debt & cleanup in several classes, and it was cleaner to do this as standalone release.

• Migrated all asserts to use the new class System.Assert
• Added the System namespace to all of the below references - this helps avoid naming collisions in orgs that have custom classes with 1 (or more) of these names
  • Classes
    • Exception, AuraHandledException, CalloutException, and IllegalArgumentException
    • Http, HttpRequest, and HttpResponse
    • SObjectAccessDecision
    • UserInfo
  • Enums
    • AccessType
    • LoggingLevel