feat(iaas): support for v2 API (#1070)

relates to STACKITTPR-313

templates/resources/network_area_route.md.tmpl

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+---
+# generated by https://github.com/hashicorp/terraform-plugin-docs
+page_title: "{{.Name}} {{.Type}} - {{.ProviderName}}"
+subcategory: ""
+description: |-
+  {{ .Description | trimspace }}
+---
+
+# {{.Name}} ({{.Type}})
+
+{{ .Description | trimspace }}
+
+## Example Usage
+
+{{ tffile (printf "examples/resources/%s/resource.tf" .Name) }}
+
+## Migration of IaaS resources from versions <= v0.74.0
+
+The release of the STACKIT IaaS API v2 provides a lot of new features, but also includes some breaking changes
+(when coming from v1 of the STACKIT IaaS API) which must be somehow represented on Terraform side. The 
+`stackit_network_area_route` resource did undergo some changes. See the example below how to migrate your resources.
+
+### Breaking change: Network area route resource (stackit_network_area_route)
+
+**Configuration for <= v0.74.0**
+
+```terraform
+resource "stackit_network_area_route" "example" {
+  organization_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  network_area_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  prefix          = "192.168.0.0/24" # prefix field got removed for provider versions > v0.74.0, use the new destination field instead
+  next_hop        = "192.168.0.0" # schema of the next_hop field changed, see below
+}
+```
+
+**Configuration for > v0.74.0**
+
+```terraform
+resource "stackit_network_area_route" "example" {
+  organization_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  network_area_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  destination = { # the new 'destination' field replaces the old 'prefix' field
+    type  = "cidrv4"
+    value = "192.168.0.0/24" # migration: put the value of the old 'prefix' field here
+  }
+  next_hop = {
+    type  = "ipv4"
+    value = "192.168.0.0" # migration: put the value of the old 'next_hop' field here
+  }
+}
+```
+
+{{ .SchemaMarkdown | trimspace }}
+