Pull backup:clean settings into config

[snipe]

This just pulls the hard-coded backup:clean settings into the env, using the same defaults that we've always used if you don't have these set.

[what-the-diff]

PR Summary

• Introduction of New Environment Variables for Backup Retention
  These environment variables were added to .env.example. They allow for the customization of the retention period for different types of backups. The variable names and their default values are as follows:

  • ALL_BACKUP_KEEP_DAYS=7: Keeps all backups for 7 days.
  • DAILY_BACKUP_KEEP_DAYS=16: Keeps daily backups for 16 days.
  • WEEKLY_BACKUP_KEEP_WEEKS=8: Keeps weekly backups for 8 weeks.
  • MONTHLY_BACKUP_KEEP_MONTHS=4: Keeps monthly backups for 4 months.
  • YEARLY_BACKUP_KEEP_YEARS=2: Keeps yearly backups for 2 years.
  • BACKUP_PURGE_OLDEST_AT_MEGS=5000: Initiates a purge of the oldest backups when the total backup size exceeds 5000 megabytes.

• Configuration File Updated for Backup Retention Period
  The configuration file config/backup.php was updated to use the new environment variables rather than the previous hardcoded values. This allows for more flexibility, as the retention periods can be adjusted more easily based on individual needs or requirements.