When Akismet is installed as an mu-plugin, don't show prompt to set it up

#### Steps to reproduce the issue

1. Install Akismet as an mu-plugin and connect Jetpack user
2. Go to Jetpack Dashboard `wp-admin/admin.php?page=jetpack#/dashboard`
3. Scroll down to `Anti-spam` feature card below stats
4. See `Your Jetpack plan provides anti-spam protection through Akismet. Click 'set up' to enable it on your site.`

#### What I expected
I would expect to see the message confirming that Akismet is installed and a [prompt to view stats](https://github.com/Automattic/jetpack/blob/576768587a60e04317b94aa22cc4687dcf4dc3e1/_inc/client/my-plan/my-plan-body.jsx#L333-L341).

#### What happened instead
We display a message to set up install / set up Akismet:

*if image does not load, please click on this link: https://d.pr/i/z7W6w2

#### Possible solutions
I haven't investigated deeply, but it looks like perhaps we could include the result of `get_mu_plugins()` in the data returned by [this function](https://github.com/Automattic/jetpack/blob/de7438643c0d3c01176e6f7b31d1401aca0674d8/_inc/lib/plugins.php#L180)?