Synapse emits invalid `m.room.power_levels` events

### Description

Containing unstable non-prefixed `historical`

The historical field is from MSC2716 which has not yet reached FCP so at best this should be behind an opt-in flag, definitely should not have polluted countless rooms with a field which lacks namespacing.

matrix.org for example does not signal in `/versions` that it has `org.matrix.msc2716` support - yet does this anyway.

The MSC states namespacing for this field is not needed because it relies on an experimental room version `org.matrix.msc2716` & `org.matrix.msc2716v2` which matrix.org does not even claim it supports.


### Steps to reproduce

1. create a new room with a stock Synapse
2. inspect `m.room.power_levels` event

### Homeserver

matrix.org

### Synapse Version

1.128.0rc1 (b=matrix-org-hotfixes,a8a6401b2f)

### Installation Method

I don't know

### Database

matrix.org

### Workers

I don't know

### Platform

matrix.org

### Configuration

_No response_

### Relevant log output

```shell
Unknown
```

### Anything else that would be useful to know?

_No response_