FCM: `apns.fcmOptions.imageUrl` and `notification.imageUrl` not rendering images for < iOS 18 and < ipadOS 18

### [REQUIRED] Step 2: Describe your environment

* Operating System version: < iOS 18, < ipadOS 18 (client), Ubuntu 14.04.6 LTS (server)
* Firebase SDK version: 12.0.0
* Firebase Product: Cloud messaging (FCM)
* Node.js version: 16 (LTS)
* Yarn version: 1.22.22

### [REQUIRED] Step 3: Describe the problem

1. Images do not render on iOS < 18 and ipadOS < 18 devices, when sending a payload with `apns.fcmOptions.imageUrl` or `notification.imageUrl`. I thought it might be related to image size so I sent an image with a size of 400 bytes but still no luck.

2. Slightly unrelated, but the docs state to use `apns.fcmOptions.image` ([source](https://firebase.google.com/docs/cloud-messaging/android/send-image#build_the_send_request)) whilst the TypeScript interface declares `apns.fcmOptions.imageUrl` ([code](https://github.com/firebase/firebase-admin-node/blob/83585498d9bd48b9ba715a882a3b9c951ca8422d/etc/firebase-admin.messaging.api.md#L69)). I've tried setting an ignore rule for the compiler to use `apns.fcmOptions.image` in the payload, whilst this also renders images for iOS 18 and ipadOS 18, it still doesn't render for iOS 17 and ipadOS 17 devices.

#### Steps to reproduce:

What happened? How can we make the problem occur?
This could be a description, log/console output, etc.

1. Create notification payload with image url set as `apns.fcmOptions.imageUrl` or `notification.imageUrl`
2. Send notification (using `sendEachForMulticast()`)
3. Observe received notification on iOS 18 and ipadOS 18 devices, image successfully displayed on both
4. Observe received notification on iOS 17 and ipadOS 17 devices, image not displayed on either

Example: Successful notification on ios 18 with image

#### Relevant Code:

Example payload
```ts
{
apns: {
fcmOptions: {
imageUrl: "imageUrl"
},
payload: {
aps: {
alert: {
body: "body"
},
mutableContent: true
}
}
}
}
```