Fixing wrong index failure due autoscaling condition for appgw

All Submissions:

---

• [YES] Have you added an explanation of what your changes do and why you'd like us to include them? Already added in !700 (merged)
• [YES] I have updated the documentation accordingly. docs/upgrade/appgw-istiogw-migration.md
• [YES] My code follows the code style of this project.

Current Behavior or Linked Issues

---

Current error (while using feature.autoscaling_enabled):

Error: Invalid index

  on diagnostics.tf line 101, in resource "azurerm_monitor_diagnostic_setting" "istio_gw_diagnostics":
 101:   target_resource_id         = module.istio_appgateway[count.index].id
    |----------------
    | count.index is 0
    | module.istio_appgateway is object with 7 attributes

The given key does not identify an element in this collection value.

Does this introduce a breaking change?

---

• [NO] Introduces fix

Other information

---

• Ref: !700 (merged)
• Ref: #236 (closed)