Paywall Block: Add gradient to the last paragraph

### Impacted plugin

Jetpack

### What

Adding a gradient to the last paragraph will give the impression that the content continues after the paywall. As it is right now, it seems like they've already read the entire post.

### How

Start by locating the Jetpack paywall block and reviewing the provided screenshot to understand the intended visual treatment. Done means the last paragraph shows a gradient that makes the continuation of content beyond the paywall apparent.

Written by the indexing model from the issue text.