[Az.Aks] folder name ".azure" is hardcoded.

### Description

In code piece: https://github.com/Azure/azure-powershell/blob/3318943634bab88fb39cfc7f150650f23502ddc9/src/Aks/Aks/Commands/KubeCmdletBase.cs#L120

The Azure PowerShell config folder name is hard coded to "**.azure**", which is not correct.
On Windows, it's case-insensitive. It's ok.
But on Linux, it's case-sensitive. It's not ok.

### Issue script & Debug output

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### Environment data

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### Module versions

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### Error output

```PowerShell
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