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

[jackbatzner]

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) 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.

[jackbatzner]

Can you provide some guidance on this one @fabiocav , @mattchenderson , @nzthiago ?