Marketing -> Traffic settings: Add correct plan slug to upsell URL

**Task Details**
As of https://github.com/Automattic/wp-calypso/pull/53196 the plan slug for free Atomic sites, for the Advanced SEO and Google Analytics nudge URLs, will be `jetpack_business` and `jetpack_premium`, instead of `business-bundle` and `value_bundle`. We should return the least expensive plan each site can buy in order to get those features, which in the case of free Atomic is Business and Premium plans, not Jetpack plans.

To fix this we should return the correct plan slug from the sites/site/features endpoint. See D63248-code#1307107

Start at the sites/site/features endpoint and review how it selects plan slugs for the Advanced SEO and Google Analytics nudge URLs. Done means free Atomic sites return the Business and Premium plan slugs rather than the Jetpack plan slugs described in the task.

Written by the indexing model from the issue text.