Chat exports miss thread content

### Steps to reproduce

1. Generate chat export from a room that contain [threads](https://element.io/blog/introducing-threads-in-beta/)

### Outcome

#### What did you expect?

to include content from threads in the exported chat

#### What happened instead?

exported chat does not include content from threads

I created a room to reproduce it easily with some messages. For example, only the root message, say "thread1" is in the export, but not the messages inside the thread such as this capture:


[Element - test export threads - Chat Export - 2024-03-30T18-50-32.055Z.zip](https://github.com/element-hq/element-web/files/14813032/Element.-.test.export.threads.-.Chat.Export.-.2024-03-30T18-50-32.055Z.zip)

I think logs are not needed, I think the export chat was developed prior to threads inclusion and that it was not adapted to the new situation

### Operating system

_No response_

### Browser information

_No response_

### URL for webapp

_No response_

### Application version

_No response_

### Homeserver

_No response_

### Will you send logs?

No