[Premium] App Service Plan capacity property no longer being updating on ARM Template deploys

**Describe the bug**
We use ARM Templates to deploy our Azure Resources. Teams have recently identified that when deploying a change to the App Service Plan's `Sku.Capacity` value that it is no longer updating. These App Service Plans are used by an Azure Function. When looking in the Azure Portal we are now seeing that we are being forced to use the `Always Ready Count` property.


This seems like a breaking change in the service layer that should be backwards compatible with this version. Teams may not be ready to upgrade to the new Function version and should be allowed to use the `sku.capacity` property still. The Release Notes for the `Microsoft.Web\sites` ([2023-12-01](https://learn.microsoft.com/en-us/azure/templates/microsoft.web/change-log/sites#2023-12-01)) don't mention this as a breaking change also.

**To Reproduce**
Steps to reproduce the behavior:
Deploy an ARM Template using `Microsoft.Web\serverFarms` api version `2018-02-01` and `Microsoft.Web\sites` api version `2021-02-01` Function App and App Service Plans and attempt to change the `sku.capacity` value. Notice it is not being updated.

** Will create a repro ARM Template and update here**

**Expected behavior**
Teams using the `sku.capacity` setting to update always ready count should be able to use this still if they are on an old version of the API version.