Fixed importer audit date validation issue

[snipe]

We were using an invalid field name for one of the field maps in the importer. I also pulled the date time validation from last_audit_date, since we use an accessor and a mutator nowadays, so we'll always get it as a date time or a null.

[what-the-diff]

PR Summary

• Correction in Asset Data Import
  The original code in the AssetImporter.php was incorrectly looking at the next_audit_date for the value of asset_eol_date. This has now been corrected to pull the asset_eol_date from its correct column in the CSV file.

• Changes to Asset Audit Date Validation
  The validation rule for last_audit_date in Asset.php now allows for more flexible formats by removing the specific date and time structure. This change will make the system more robust, accommodating varying date inputs.