routing.yml file doesn't work
You're saying that you have this in your routing.yml file:

user_profile_update.callback:
  path: '/user-update'
  defaults:
    _controller: '\Drupal\user_profile\Controller\ProfileUpdateController::updateUser'
  requirements:
    _user_is_logged_in: 'TRUE'
    _permission: 'access content'


You're still calling updateUser and not updateUser123

Your controller should be looking like this:

class ProfileUpdateController extends ControllerBase {

  public function updateUser123() {
  //your code here
  }

}


And now you'll need to call the method:

user_profile_update.callback:
  path: '/user-update'
  defaults:
    _controller: '\Drupal\user_profile\Controller\ProfileUpdateController::updateUser123'
  requirements:
    _user_is_logged_in: 'TRUE'
    _permission: 'access content'


Also I recommend clearing your cache with Drush.
This will empty all caches and rebuild the data required for Drupal to execute the page request