Fixed small deprecation warnings for PHP8.2

[snipe]

This should take care of some deprecations warnings in the Asset model regarding depreciations

[what-the-diff]

PR Summary

• Refinement of Asset Assignment Type Retrieval
  The pull request includes a change to how asset assignment types are retrieved in the system. Previously, the type (e.g., 'monitor', 'computer', etc.) of an assigned asset was returned exactly as it exists in the system. The proposed modification aims to standardize these assignment types by making them all lowercase, which could potentially improve data consistency and searchability. If no assignment type exists for the asset, a 'nil' value will be returned instead.

• Improvement of Depreciation Date Calculation
  The pull request introduces an additional check for the depreciation date calculation method within depreciable assets. Here, both the purchase date and depreciation details are required for accurate calculation. If both of these values are present, the system will calculate the depreciation date accurately by adding the defined depreciation time (in months) to the purchase date. In the case where either or both of these essential parameters are missing, the method will now return a 'nil' value, alerting the user that depreciation determination wasn't possible due to missing information.