Support .yml extension for the config file (#162)

## Overview/Summary

The intent seems to be that both `.yaml` and `.yml` would be valid
configuration file extensions. However, in the current code, a `.yml`
file extension would attempted to be parsed as JSON.

## This PR fixes/adds/changes/removes

1. Azure/ALZ-PowerShell-Module#153 

### Breaking Changes

N/A

## As part of this Pull Request I have

- [x] Checked for duplicate [Pull
Requests](https://github.com/Azure/alz-terraform-accelerator/pulls)
- [x] Associated it with relevant
[issues](https://github.com/Azure/alz-terraform-accelerator/issues), for
tracking and closure.
- [x] Ensured my code/branch is up-to-date with the latest changes in
the `main`
[branch](https://github.com/Azure/alz-terraform-accelerator/tree/main)
- [x] Performed testing and provided evidence.
- [x] Updated relevant and associated documentation.

---------

Co-authored-by: Jared Holgate <[email protected]>

templates/complete/locals.tf

@@ -6,7 +6,7 @@ locals {
   const_yml             = "yml"
 }
 locals {
-  config = (local.config_file_extension == local.const_yaml ?
+  config = (local.config_file_extension == local.const_yaml || local.config_file_extension == local.const_yml ?
     yamldecode(templatefile("${path.module}/${local.config_file_name}", local.config_template_file_variables)) :
     jsondecode(templatefile("${path.module}/${local.config_file_name}", local.config_template_file_variables))
   )

templates/complete_vnext/locals.tf

@@ -6,7 +6,7 @@ locals {
   const_yml             = "yml"
 }
 locals {
-  config = (local.config_file_extension == local.const_yaml ?
+  config = (local.config_file_extension == local.const_yaml || local.config_file_extension == local.const_yml ?
     yamldecode(templatefile("${path.module}/${local.config_file_name}", local.config_template_file_variables)) :
     jsondecode(templatefile("${path.module}/${local.config_file_name}", local.config_template_file_variables))
   )