Drupal 8.x

Is it possible for upgrading from Drupal 8.7.14 to directly Drupal 9.x, instead of upgrading it to 8.8.x or 8.9.x?

I´ve used  \Drupal::formBuilder()->getForm() to pass arguments to form and get it, but when I click on submit button , It appears The website encountered an unexpected error. Please try again later. I think that I get the form but not the methods of validation and submit of own form.

I have a button in header view of a content type, It is to add a entity with the information of the current node, that is to say, 

I am trying to subscribe to the commerce_payment.filter_payment_gateways event, but it is not working.  I have been able to subscribe to other events successfully (commerce_product.commerce_product_variation.load).  I notice that other events have 3 parts (.) (module, component, method/action):

• commerce_product
• commerce_product_variation
• load

whereas this event only has two:

• commerce_payment
• filter_payment_gateways

Could that be the issue?

This is my subscriber code... basic as it gets.

It has been a while since I have installed Drupal and things have changed.  Just installed Drupal 8.8.8.  Post installation, I went to install a theme from the site and noticed I am missing the Install new theme button.  From the Report > Status page I can see a warning that I should enable Update Manager.  From the Module page it looks that this module is enabled, or at least the box is checked?