feat!(next/image): change default `Content-Disposition` to `attachment` (#65631)
### BREAKING CHANGE
This changes the behavior of the default image `loader` so that
[`Content-Disposition`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Disposition#as_a_response_header_for_the_main_body)
header is now `attachment` for added protection since the API can serve
arbitrary remote images.
The new default value, `attachment`, forces the browser to download the
image when visiting directly. This is particularly important when
`dangerouslyAllowSVG` is true. Most users will not notice the change
since visiting pages won't behave any differently, only visiting images
directly.
Users can switch back to the old behavior by configuring `inline` in
next.config.js
```js
module.exports = {
images: {
contentDispositionType: 'inline',
},
}