Drupal 9 compatibilty

After looking at this a bit I think I see the following:

* Same Page Preview benefits from the work that was done with the off-canvas dialog.
* Specifically the all: reset that applies to the containing element
* the css the node module's node.preview.css is exactly the same between drupal 9 and 10.1

It looks like the issue we're seeing in Drupal 9 will require quite a bit of work to fix. Maybe we should just say same_page_preview requires Drupal 10 and above?

So are we done here?

Rebased MR and looks like dropping support for Drupal 9 and only support Drupal 10, am I, right?.

Appreciate the work here, but now that we have introduced Single File Components into 2.1.x I think that means that we won't be supporting Drupal 9 for this module. Marking as wontfix.