Pinned message sidebar only shows the first 100 pinned messages

### Steps to reproduce

1. Where are you starting? What can you see?
room timeline
2. What do you click?
pinned messages
3. More steps…

### Outcome

#### What did you expect?
to see all pinned messages
#### What happened instead?
only the first 100 are shown

this also happens in the browser version (tested in firefox)

### Operating system

Arch Linux

### Application version

Element version: 1.12.9 Crypto version: Rust SDK 0.16.0 (95b5e83), Vodozemac 0.9.0

### How did you install the app?

pacman -S element-desktop

### Homeserver

matrix.org

### Will you send logs?

No

Start by reproducing the issue in the pinned messages sidebar with more than 100 pinned messages, using the Element web client in Firefox or the desktop app. Trace how the sidebar loads pinned messages and identify where the first-100 limit is introduced; done means all pinned messages are displayed.

Written by the indexing model from the issue text.