Related posts block: does not render well in emails and Reader

### Impacted plugin

Jetpack

### Quick summary

Related posts block needs styles for the emails and reader:

### Email

### WP.com Reader

### Jetpack apps reader

### Steps to reproduce

1. Have a blog with few posts and subscriptions enabled (all wp.com simple sites)
2. Subscriber to your blog
3. Create a blog post with "related posts" block
4. Send it
5. See the email

### A clear and concise description of what you expected to happen.

_No response_

### What actually happened

_No response_

### Impact

Some (< 50%)

### Available workarounds?

No but the platform is still usable

### Platform (Simple and/or Atomic)

Simple

### Logs or notes

_No response_
```[tasklist]
### Tasks
- [ ] Rendering in WP.com reader
- [ ] Rendering in Jetpack/WP.com Emails
- [ ] Rendering in Jetpack app reader (iOS/Android)
```