Scrolling through NotifPanel should make the red dot go away

### Your use case

#### What would you like to do?

I re-added a red dot for missed messages on NotifPanel in https://github.com/matrix-org/matrix-react-sdk/pull/9242.

However, when you scroll back through NotifPanel, you expect the dot to disappear. It doesn't.

#### Why would you like to do it?

So you know if you've read up on recent notifs on NotifPanel or not.

#### How would you like to achieve it?

Probably clear the dot whenever you open NotifPanel, and then re-add it when new notifs come in.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_