Own knock or invite event in knock_state or invite_state in /sync is not stripped

This issue has been migrated from [#14919](https://github.com/matrix-org/synapse/issues/14919).

---

### Description

https://github.com/matrix-org/synapse/blob/19c0e55ef7742d67cff1cb6fb7c3e862b86ea788/synapse/rest/client/sync.py#L345 does not strip the invite event it is pulling the invite_state from. The same thing happens for knocking.

This is then sent to clients via /sync. According to the spec all events in invite/knock_state should be stripped. There are benefits to the client having access to the timestamp of the invite however, so some might be relying on this, but strictly speaking this is a spec violation and looks like a bug.

Spec: https://spec.matrix.org/v1.5/client-server-api/#get_matrixclientv3sync

### Steps to reproduce

- get invited or knock a room
- look at the sync response

### Homeserver

neko.dev

### Synapse Version

1.74

### Installation Method

Other (please mention below)

### Database

postgres, single

### Workers

Multiple workers

### Platform

Gentoo

### Configuration

Can be reproduced on various configurations

### Relevant log output

```shell
Screenshots and link to the offending code above, no logs about it, since it is a spec compliance issue.
```

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

_No response_

Start at synapse/rest/client/sync.py around line 345 and reproduce the invite and knock responses through /sync. Compare the response contents with the Matrix client-server specification; done means the invite_state and knock_state no longer include the originating event while the rest of the response remains correct.

Written by the indexing model from the issue text.