fix: availability zone data type (#275)

# Pull Request

## Issue

Azure/ALZ-Bicep#939

## Description

Fix the data type of availability zones as Bicep is expecting an array
of strings

## License

By submitting this pull request, I confirm that my contribution is made
under the terms of the projects associated license.

src/ALZ/Private/Config-Helpers/Get-AzureRegionData.ps1

@@ -25,7 +25,7 @@ module "regions" {
 locals {
   regions = { for region in module.regions.regions_by_name : region.name => {
     display_name = region.display_name
-    zones        = region.zones == null ? [] : region.zones
+    zones        = region.zones == null ? [] : [for zone in region.zones : tostring(zone)]
     }
   }
 }