fix: modernize cloud-init user password configuration

Replace deprecated chpasswd.list with plain_text_passwd in user configuration.
The chpasswd.list method is deprecated in newer cloud-init versions and
plain_text_passwd is the recommended approach for setting user passwords.

This change:
- Removes the deprecated chpasswd section
- Adds plain_text_passwd field to the user configuration
- Maintains the same password (torrust123) for compatibility
- Eliminates deprecation warnings in cloud-init logs

Author: josecelano

infrastructure/cloud-init/user-data.yaml.tpl

@@ -27,15 +27,10 @@ users:
     sudo: ["ALL=(ALL) NOPASSWD:ALL"]
     shell: /bin/bash
     lock_passwd: false
+    plain_text_passwd: torrust123
     ssh_authorized_keys:
       - ${ssh_public_key}
 
-# Set password using chpasswd (most reliable method)
-chpasswd:
-  list: |
-    torrust:torrust123
-  expire: false
-
 # Enable SSH password authentication for debugging
 ssh_pwauth: true