Valid config and update how configuration is saved

I've created a merge request to simplify the cacheexclude_node_types config format. Please review.
Thank you!

appear to be missing the test of the ticket.

So no update hook or schema increase.

To get credit the ticket should be attempted not just a single part please

@smustgrave I have added update hook. Kindly review.

Thank you!

The update hook number probably should be 21001 to match the version it would be landing

And schema validation part is missing. EntityBundleExists maybe?