Jetpack Scan: add support for VaultPress users without the security feature

[v18]

When adding Jetpack Scan on a site that has a VaultPress plan without scanning, Jetpack Scan accepts the SSH credentials silently and does not enable scanning.

Instead, Jetpack Scan should work for all sites with VaultPress plans that do not have scanning. The current VaultPress plans that do not have scanning are:
Backup Bundle
Lite
Basic

Steps to reproduce:

1. Add one of the non-security VaultPress plans to a site
2. Install the VaultPress plugin on the site
3. Add malicious files to the site's server (in wp-content for example)
4. Activate Jetpack Start - see that it's activated
5. See that no Jetpack Scan emails are sent even after the first scan is complete

[stale]

This issue has been marked as stale because it hasn't been updated in a while. It will be closed in a week. If you would like it to remain open, can you please comment below and see what you can do to get things moving with this issue? Thanks! 🙏