Form block: mention Akismet protecting the form from spam in the sidebar

When [Akismet](https://wordpress.org/plugins/akismet/) is enabled, the form is protected from submission spam. We could mention this in the Form block's sidebar.

We cannot easily check if Akismet is installed or enabled, so ideally this would just be a generic sentence that works in both scenarios.

Not sure what it would require to check plugins state in the block, but it's definitely some more work. WP REST API doesn't seem to [be aware of plugins](https://developer.wordpress.org/rest-api/reference/) yet.

came up at p1HpG7-6mq-p2#comment-30721


cc @jessefriedman

Start by locating the Form block and its sidebar controls in the Jetpack repository, then inspect how nearby sidebar text is defined. Confirm whether the issue can be handled without checking plugin state, and consider the existing Akismet and WordPress REST API references. Done means the sidebar communicates the generic spam-protection benefit in both scenarios.

Written by the indexing model from the issue text.