synapse allows sending of unecrypted messages to encrypted rooms

### Description

When posting a message wtih m.room.message type to synapse to a room that is encrypted, synapse accepts the message and stores it.

### Steps to reproduce

Post an unencrypted message to an encrypted room.

### Homeserver

any

### Synapse Version

1.115.0

### Installation Method

Docker (matrixdotorg/synapse)

### Database

postgresql

### Workers

Single process

### Platform

Kubernetes

### Configuration

-

### Relevant log output

```shell
-
```

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

Expected behaviour is after m.room.encryption unencrypted messages are rejected.