Cannot send Custom State event

### Steps to reproduce

Hello I'm running into an issue while trying to set the retention.
I'm using Element Android ver 1.5.26
I keep getting 'Malformed JSON' when I try to send the m.room.retention event via dev tools

Steps:
1. Go to the room settings and open Dev Tools-> Send custom event
2. Type: m.room.retention content: {"max_lifetime": 8640000}.
3. Send event

### Outcome

#### What did you expect?
State event sent successfully and room state updated

#### What happened instead?
Receives error "Malformed JSON
I've searched around and validated the JSON string format but cannot get it to work. Any help would be greatly appreciated

### Your phone model

Pixel 7

### Operating system version

Android 12

### Application version and app store

1.5.26

### Homeserver

Matrix/Synapse homeserver

### Will you send logs?

Yes

### Are you willing to provide a PR?

Yes