Map Block: provide a better UI fallback on wpcom sites, where an API key is provisioned

#### Steps to reproduce the issue

On a WordPress.com site, a Mapbox API key is provided to you so you don't have to create one for yourself:
https://github.com/Automattic/jetpack/blob/f8969425fdf4fe2aa08a797ea86d321238436ba4/projects/plugins/jetpack/_inc/lib/class-jetpack-mapbox-helper.php#L59-L64

However, the UI does not show that API key:

It is fine not to show the key, but it would be nice if the UI would indicate that a key has already been provisioned to you. Alternatively, we could remove that UI element altogether:
https://github.com/Automattic/jetpack/blob/dcec8da3dec6e37868e7f4b4793567705a81dd73/projects/plugins/jetpack/extensions/blocks/map/controls.js#L176-L209

Before we make such a change, we should consider previous discussions and PRs about this, where we deliberately chose to display the field on WordPress.com sites: #14597.

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Related issue: #20591

Start with class-jetpack-mapbox-helper.php and extensions/blocks/map/controls.js, then review the discussion in #14597 and related issue #20591. The work is done when the Map block UI clearly handles the provisioned WordPress.com API key without exposing it unnecessarily, according to the chosen approach.

Written by the indexing model from the issue text.