Response from /knock after /invite does not conform to the specification

This issue has been migrated from [#16735](https://github.com/matrix-org/synapse/issues/16735).

---

### Description

The specification allows re-knocking after having been invited (https://spec.matrix.org/v1.9/client-server-api/#mroommember).

However, Synapse returns a different response if you re-knock after having been invited:

```
{'errcode': 'M_FORBIDDEN', 'error': 'You are already invited to this room'}
```

### Steps to reproduce

- Create a room
- Invite user to a room
- Knock on the room as an invitee

### Homeserver

local

### Synapse Version

v1.93.0

### Installation Method

Docker (matrixdotorg/synapse)

### Database

SQLite

### Workers

Single process

### Platform

Docker Container (`matrixdotorg/synapse:v1.93.0`)

### Configuration

We use the default homeserver options, but with ratelimiting disabled.

### Relevant log output

```shell
n/a
```

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

_No response_