chore(docs): use new 'default_region' provider attribute (#793)

instead of deprecated 'region' attribute

docs/index.md

@@ -6,27 +6,27 @@ The STACKIT Terraform provider is the official Terraform provider to integrate a
 
 ```terraform
 provider "stackit" {
-  region = "eu01"
+  default_region = "eu01"
 }
 
 # Authentication
 
 # Token flow
 provider "stackit" {
-  region                = "eu01"
+  default_region        = "eu01"
   service_account_token = var.service_account_token
 }
 
 # Key flow
 provider "stackit" {
-  region              = "eu01"
+  default_region      = "eu01"
   service_account_key = var.service_account_key
   private_key         = var.private_key
 }
 
 # Key flow (using path)
 provider "stackit" {
-  region                   = "eu01"
+  default_region           = "eu01"
   service_account_key_path = var.service_account_key_path
   private_key_path         = var.private_key_path
 }