Synapse Expires `og:image` for URL preview despire not being allowed by spec.

### Description

The `og:image` URL returned from [` /_matrix/client/v1/media/preview_url`](https://spec.matrix.org/v1.15/client-server-api/#get_matrixclientv1mediapreview_url) expires.

This is not allowed by spec and causes breakage in clients.

### Steps to reproduce

1. Send a link to an image.
2. See that the URL preview appears.
3. Wait (an hour usually does it)
4. Click the image to load the full-size.

## Expected

The image loads.

## Observed

The image URL returns 404.

### Homeserver

kevincox.ca

### Synapse Version

1.133.0

### Installation Method

Other (please mention below)

### Database

PostgreSQL, one server, never squite, never restored from backup

### Workers

Single process

### Platform

NixOS

### Configuration

URL previews

### Relevant log output

```shell
none
```

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

Originally reported to element-web here https://github.com/element-hq/element-web/issues/30238 where they pointed out that this is because the server is not following the spec.

This is probably also a spec bug, as it doesn't provide any mechanism for expiry which should probably be considered/allowed on basically every type of content as we can't expect everything to be stored forever.