Incorrect syntax for user mentions

### Steps to reproduce

Mention a user from Element iOS nightly. For example: "I DMed tpb about what’s going on but I suspect I’m just talking to a bot/service. 🤔"

### Outcome

#### What did you expect?

This event was sent:

According to the spec:

* https://spec.matrix.org/v1.7/client-server-api/#mroommessage-msgtypes

Something like this should be sent:

```json
{
"content": {
"body": "I DMed tpb about what’s going on but I suspect I’m just talking to a bot/service. 🤔",
"format": "org.matrix.custom.html",
"formatted_body": "I DMed tpb about what’s going on but I suspect I’m just talking to a bot/service. 🤔",
"msgtype": "m.text"
},
...
}
```

allowing matrix-appservice-irc to translate it to:

```

I DMed tpb about what’s going on but I suspect I’m just talking to a bot/service. 🤔
```

#### What happened instead?

```json
{
"content": {
"body": "I DMed [tpb]() about what’s going on but I suspect I’m just talking to a bot/service. 🤔",
"format": "org.matrix.custom.html",
"formatted_body": "I DMed tpb about what’s going on but I suspect I’m just talking to a bot/service. 🤔",
"msgtype": "m.text"
},
"origin_server_ts": 1690466951796,
"room_id": "!LhJjrGmNCUyJajtYYf:catircservices.org",
"sender": "@jevinskie:matrix.org",
"type": "m.room.message",
"unsigned": {
"age": 4266509
},
"event_id": "$30s5H_bel7LYbK_APhXlljW-hXq1bgSiQKjkzeLe-4Q",
"user_id": "@jevinskie:matrix.org",
"age": 4266509
}
```

https://matrix.to/#/!LhJjrGmNCUyJajtYYf:catircservices.org/$30s5H_bel7LYbK_APhXlljW-hXq1bgSiQKjkzeLe-4Q?via=matrix.org&via=catircservices.org

which causes the following to be sent to IRC through matrix-appservice-irc:

```
I DMed [tpb]() about what’s going on but I suspect I’m just talking to a bot/service. 🤔
```

(which then triggers URL snippet fetching, which is very noisy)

### Your phone model

_No response_

### Operating system version

_No response_

### Application version

nightly, 1.10.14 (20230621144257)

### Homeserver

_No response_

### Will you send logs?

No