Update Azure template

- Remove Ubuntu 18.04, which is EOL after May 2023.

azure/azuredeploy.json

@@ -25,12 +25,11 @@
       "type": "string",
       "allowedValues": [
         "ubuntu22.04",
-        "ubuntu20.04",
-        "ubuntu18.04"
+        "ubuntu20.04"
       ],
       "defaultValue": "ubuntu22.04",
       "metadata": {
-        "description": "OS to use. Ubuntu 22.04, 20.04 or 18.04."
+        "description": "OS to use. Ubuntu 22.04 or 20.04."
       }
     },
     "VMSize": {
@@ -73,12 +72,6 @@
       "sku": "20_04-lts",
       "version": "latest"
     },
-    "ubuntu18.04": {
-      "publisher": "Canonical",
-      "offer": "UbuntuServer",
-      "sku": "18.04-LTS",
-      "version": "latest"
-    },
     "installScriptURL": "https://raw.githubusercontent.com/hwdsl2/setup-ipsec-vpn/master/azure/install.sh",
     "installCommand": "[concat('sh install.sh ', variables('quote'), parameters('preSharedKey'), variables('quote'), ' ', variables('quote'), parameters('username'), variables('quote'), ' ', variables('quote'), parameters('password'), variables('quote'))]"
   },