COP DEM STAC Items don't work in ODC STAC

Notebook example here: https://gist.github.com/alexgleith/0d705d526b596b7a00fbfa589b5d3fd0

Issue is storing the `proj:epsg` field as a float:

```
'proj:epsg': 4326.0,
```

The [spec](https://github.com/stac-extensions/projection) says it should be `int`