Fixed maintenances patch endpoint

[snipe]

This should resolve an unreported issue on the (existing but not documented) PATCH method on the maintenances endpoint. Previous behavior was that it would blank out fields if they were not present in the request (which is not what PATCH should be doing.)

[shortcut-integration]

This pull request has been linked to Shortcut Story #24315: PATCH method for updating asset maintenances overwrites values with null if payload doesn't include them.

[what-the-diff]

PR Summary

• Simplified Data Assignment Process
  In the controller for "Asset Maintenances", the PR has introduced a more efficient way of assigning data to the 'AssetMaintenance' model using an operation called fill(). They've ensured that not all data can be assigned directly to protect from any unauthorized access by specifying a fillable property.

• Enhanced Date Validations
  In 'AssetMaintenance', the start date and completion date fields now have additional checks to ensure the date is specified correctly. This helps in maintaining consistent datetime data and minimizes manual errors.

• Language Support Updates
  More translation words have been added to the 'general' language file. It includes phrases for scenarios where an item is not available and permission for a particular action is denied. It also extends the support for the 'edit' operation, making the application more understandable and user-friendly.