Dashboard: add an option to remove the Jetpack dashboard for subscribers

#### Is your feature request related to a problem? Please describe.
A [user requested](https://wordpress.org/support/topic/subscriber-users-are-able-to-see-jetpack-dashboard-tab/) the ability to disable the Jetpack dashboard for subscribers.

#### Describe the solution you'd like
We should consider adding a setting that allows site administrators to remove the dashboard for subscribers.

We should also provide information about why they may want to enable the dashboard for subscribers. The dashboard and Jetpack connection allows subscribers to use SSO, receive notifications, and obtain a quick link for adding the site to their WordPress mobile app.

#### Describe alternatives you've considered
n/a

#### Additional context
* p9dueE-2qU-p2
* https://github.com/Automattic/jetpack/pull/18763, which restricts the first connection to admins.

The issue does not identify files, tests, or entry points. Start by tracing how the Jetpack dashboard is exposed to subscriber users and how administrator settings control access; done means administrators can disable the dashboard for subscribers while the issue's stated SSO, notification, mobile-app link, and explanatory information remain addressed.

Written by the indexing model from the issue text.