fix(updatenotification): spread the use of new iappconfig

[ArtificialOwl]

This PR fix a regression from #43967 and #43454 where new appconfig's api were used to type values while the old API was still used in some other place

Context

It has been decided that once a config value is set as anything other than mixed (default type for config value pre-29) it cannot be set back to mixed.

Meaning that once IAppConfig::setValueString() have been called on a config key, any line of code using the old API to set a value for this config key would returns an exception like this one:

{
  "reqId": "aOMsS5ZTlMntl6lNG1g8",
  "level": 3,
  "time": "2024-03-12T15:34:05+00:00",
  "remoteAddr": "127.0.0.1",
  "user": "cult",
  "app": "index",
  "method": "GET",
  "url": "/index.php/settings/admin/overview",
  "message": "conflict between new type (mixed) and old type (integer)",
  "userAgent": "Mozilla/5.0 (X11; Linux x86_64; rv:122.0) Gecko/20100101 Firefox/122.0",
  "version": "29.0.0.7",
  "exception": {
    "Exception": "OCP\\Exceptions\\AppConfigTypeConflictException",
    "Message": "conflict between new type (mixed) and old type (integer)",
    "Code": 0,
    "Trace": [
      {
        "file": "/home/maxence/sites/nc29/nextcloud/lib/private/AppConfig.php",
        "line": 1340,
        "function": "setTypedValue",
        "class": "OC\\AppConfig",
        "type": "->"
      },
      {
        "file": "/home/maxence/sites/nc29/nextcloud/lib/private/AllConfig.php",
        "line": 208,
        "function": "setValue",
        "class": "OC\\AppConfig",
        "type": "->"
      },
      {
        "file": "/home/maxence/sites/nc29/nextcloud/lib/private/Updater/VersionCheck.php",
        "line": 65,
        "function": "setAppValue",
        "class": "OC\\AllConfig",
        "type": "->"
      },
      {
        "file": "/home/maxence/sites/nc29/nextcloud/apps/updatenotification/lib/UpdateChecker.php",
        "line": 52,
        "function": "check",
        "class": "OC\\Updater\\VersionCheck",
        "type": "->"
      },
      {
        "file": "/home/maxence/sites/nc29/nextcloud/apps/updatenotification/lib/Settings/Admin.php",
        "line": 95,
        "function": "getUpdateState",
        "class": "OCA\\UpdateNotification\\UpdateChecker",
        "type": "->"
      },
      {
        "file": "/home/maxence/sites/nc29/nextcloud/apps/settings/lib/Controller/CommonSettingsTrait.php",
        "line": 129,
        "function": "getForm",
        "class": "OCA\\UpdateNotification\\Settings\\Admin",
        "type": "->"
      },
      {
        "file": "/home/maxence/sites/nc29/nextcloud/apps/settings/lib/Controller/AdminSettingsController.php",
        "line": 86,
        "function": "formatSettings",
        "class": "OCA\\Settings\\Controller\\AdminSettingsController",
        "type": "->"
      },
      {
        "file": "/home/maxence/sites/nc29/nextcloud/apps/settings/lib/Controller/CommonSettingsTrait.php",
        "line": 149,
        "function": "getSettings",
        "class": "OCA\\Settings\\Controller\\AdminSettingsController",
        "type": "->"
      },
      {
        "file": "/home/maxence/sites/nc29/nextcloud/apps/settings/lib/Controller/AdminSettingsController.php",
        "line": 71,
        "function": "getIndexResponse",
        "class": "OCA\\Settings\\Controller\\AdminSettingsController",
        "type": "->"
      },
      {
        "file": "/home/maxence/sites/nc29/nextcloud/lib/private/AppFramework/Http/Dispatcher.php",
        "line": 232,
        "function": "index",
        "class": "OCA\\Settings\\Controller\\AdminSettingsController",
        "type": "->"
      },
      {
        "file": "/home/maxence/sites/nc29/nextcloud/lib/private/AppFramework/Http/Dispatcher.php",
        "line": 138,
        "function": "executeController",
        "class": "OC\\AppFramework\\Http\\Dispatcher",
        "type": "->"
      },
      {
        "file": "/home/maxence/sites/nc29/nextcloud/lib/private/AppFramework/App.php",
        "line": 184,
        "function": "dispatch",
        "class": "OC\\AppFramework\\Http\\Dispatcher",
        "type": "->"
      },
      {
        "file": "/home/maxence/sites/nc29/nextcloud/lib/private/Route/Router.php",
        "line": 338,
        "function": "main",
        "class": "OC\\AppFramework\\App",
        "type": "::"
      },
      {
        "file": "/home/maxence/sites/nc29/nextcloud/lib/base.php",
        "line": 1050,
        "function": "match",
        "class": "OC\\Route\\Router",
        "type": "->"
      },
      {
        "file": "/home/maxence/sites/nc29/nextcloud/index.php",
        "line": 49,
        "function": "handleRequest",
        "class": "OC",
        "type": "::"
      }
    ],
    "File": "/home/maxence/sites/nc29/nextcloud/lib/private/AppConfig.php",
    "Line": 809,
    "message": "conflict between new type (mixed) and old type (integer)",
    "exception": {},
    "CustomMessage": "conflict between new type (mixed) and old type (integer)"
  }
}