Add upgrade_override block to azurerm_kubernetes_cluster main

[taustgen]

Describe your changes

Makes the upgrade_override block configurable on the kubernetes cluster.
This is important because if force upgrade is ever used via the cli the cluster cannot be modified via terraform unless that block is present.

Context:
I used the force-upgrade via the CLI to upgrade one of my clusters back in November. The upgrade_override block was detected and state updated. Now months later im starting the next upgrade cycle and this cluster cannot be updated as the upgrade_override block cannot be removed if present

Issue number

│ Error: `upgrade_override` cannot be unset
│
│   with module.aks.azurerm_kubernetes_cluster.main,
│   on ../azure-aks-azurerm/v4/main.tf line 13, in resource "azurerm_kubernetes_cluster" "main":
│   13: resource "azurerm_kubernetes_cluster" "main" {
│
╵

Checklist before requesting a review

• The pr title can be used to describe what this pr did in CHANGELOG.md file
• I have executed pre-commit on my machine
• I have passed pr-check on my machine

Thanks for your cooperation!

[lonegunmanb]

Thanks @taustgen , just about time!