Fix Authelia authentication_backend config for 4.38+ format

The working authelia-setup repo uses the Authelia 4.38+ password
hashing config format with a nested argon2 block and variant key.
The previous version used the old flat format which is rejected by
Authelia 4.39.20 validation. Fix both 24.04 and 26.04 scripts.

  Before (broken):
    password:
      algorithm: argon2id
      iterations: 3
      ...

  After (correct for 4.38+):
    password:
      algorithm: argon2
      argon2:
        variant: argon2id
        iterations: 3
        ...

https://claude.ai/code/session_01FvqXSZyk3g7rUombwLcprZ

ubuntu-post-install-24.04.sh

@@ -4983,12 +4983,14 @@ authentication_backend:
   file:
     path: /config/users.yml
     password:
-      algorithm: argon2id
-      iterations: 3
-      memory: 65536
-      parallelism: 4
-      key_length: 32
-      salt_length: 16
+      algorithm: argon2
+      argon2:
+        variant: argon2id
+        iterations: 3
+        memory: 65536
+        parallelism: 4
+        key_length: 32
+        salt_length: 16
 
 access_control:
   default_policy: deny

ubuntu-post-install-26.04.sh

@@ -4983,12 +4983,14 @@ authentication_backend:
   file:
     path: /config/users.yml
     password:
-      algorithm: argon2id
-      iterations: 3
-      memory: 65536
-      parallelism: 4
-      key_length: 32
-      salt_length: 16
+      algorithm: argon2
+      argon2:
+        variant: argon2id
+        iterations: 3
+        memory: 65536
+        parallelism: 4
+        key_length: 32
+        salt_length: 16
 
 access_control:
   default_policy: deny