Related Posts block: Make related posts of old posts refresh accordingly

As per @MichaelArestad's comment:

> One thing I did notice is that those first already published posts did not update the related posts section as more posts were added. I even tried updating it a few times, removing the blocks and readding them.

> What I would expect is that the related posts are always changing as the site grows and more content is written. I expect this would happen without having to ever go back to the older posts to "update" the related posts content.

Start by locating the Related Posts block implementation in the Jetpack plugin and trace how its post suggestions are stored and rendered for already published posts. Reproduce the behavior on an older post, then verify that its related posts change as new content is added without manually updating the post.

Written by the indexing model from the issue text.