Deprecate duplicate units class

[DannyvdSluijs]

Background

The \Picqer\Financials\Exact\Units class was added in 2017 in ad75b5c whereas the Unit (singular form) was added in 2019 (59dd598). Following the naming conventions it should be a singular form.

PR Goal

This PR updates the Units class to become an extension of Unit and adds the deprecation trigger and annotation.
Once/If we agree on if this deprecation style is acceptable for this project I can update the other duplicate classes as well. First feedback is much appreciated. Also see below about the other class duplications.

Additional info

There are multiple of these problems which can be discovered by running a bash command to find PHP files containing duplicate @see annotations in the src folder.

find ./src/Picqer/Financials/Exact -type f  -name \*.php | xargs grep '@see https://' -h | sort | uniq -c | grep '^   2'

See all duplicate @see notations

   2  * @see https://start.exactonline.nl/docs/HlpRestAPIResourcesDetails.aspx?name=DocumentsDocumentCategories
   2  * @see https://start.exactonline.nl/docs/HlpRestAPIResourcesDetails.aspx?name=HRMDivisions
   2  * @see https://start.exactonline.nl/docs/HlpRestAPIResourcesDetails.aspx?name=InventoryItemWarehousePlanningDetails
   2  * @see https://start.exactonline.nl/docs/HlpRestAPIResourcesDetails.aspx?name=LogisticsUnits
   2  * @see https://start.exactonline.nl/docs/HlpRestAPIResourcesDetails.aspx?name=ProjectTimeTransactions
   2  * @see https://start.exactonline.nl/docs/HlpRestAPIResourcesDetails.aspx?name=ReadCRMDocumentsAttachments
   2  * @see https://start.exactonline.nl/docs/HlpRestAPIResourcesDetails.aspx?name=SalesShippingMethods
   2  * @see https://start.exactonline.nl/docs/HlpRestAPIResourcesDetails.aspx?name=SyncDeleted
   2  * @see https://start.exactonline.nl/docs/HlpRestAPIResourcesDetails.aspx?name=SyncInventoryItemWarehouses
   2  * @see https://start.exactonline.nl/docs/HlpRestAPIResourcesDetails.aspx?name=SyncInventoryStockPositions
   2  * @see https://start.exactonline.nl/docs/HlpRestAPIResourcesDetails.aspx?name=SyncInventoryStorageLocationStockPositions
   2  * @see https://start.exactonline.nl/docs/HlpRestAPIResourcesDetails.aspx?name=SyncProjectTimeCostTransactions