Theme Showcase: Upsell nudge sometimes errors for non-Business Atomic

**To reproduce:**

* Switch to a site with no Business plan, but still on Atomic (*.wpcomstaging.com URL)
* Go to `/themes`
* Note the upsell above the Showcase and click on it
* You'll be brought to checkout, where hopefully you have the Business plan in your cart (good, because you shouldn't be able to get any other plan with this type of site :))
* ...but you might get a compatibility error instead:

Ideally we shouldn't show this upgrade if it's not able to be added to the cart. It's pointing to:

`/checkout/[site].wpcomstaging.com/jetpack_security_realtime`

I think the Jetpack security plan is incompatible because it's technically a free site, even though it's still using Atomic servers.

_Originally posted by @sixhours in https://github.com/Automattic/wp-calypso/issues/38807#issuecomment-901402588_

Start by reproducing the flow from `/themes` on a non-Business Atomic site, then trace the upsell link to `/checkout/[site].wpcomstaging.com/jetpack_security_realtime` and inspect the compatibility handling. Done means the incompatible upgrade is not shown, or it no longer sends the user to a checkout error.

Written by the indexing model from the issue text.