Don't show unknown/custom state events in rooms by default

### Your use case

#### What would you like to do?
Stop users from seeing confusing "custom events"

#### Why would you like to do it?
Many users don't know how Matrix works, and some users have strong adverse reactions to anything unexpected in their chat view

#### How would you like to achieve it?
Don't show any unknown event types in the room timeline, except if the user opts into viewing hidden events via a developer option (#3760)

### Have you considered any alternatives?

Users could be educated to understand Matrix better and not get scared about unknown things, but that is likely more difficult than hiding unknown events

### Additional context

_No response_

### Are you willing to provide a PR?

No