Hello MaxyS2,
Once you've restored the data from your old iPhone to the new, then you can delete the old backup if you wish. Just removing it would make it so you cannot restore that data in the future if you ever needed to reset your iPhone. Here's some more info on clearing that storage: View and manage iCloud device backups - Apple Support
You can view and manage your iCloud backups from an iPhone, iPad, iPod touch, Mac, or Windows computer.
If you want to customize which apps and data are backed up, see Back up your iPhone, iPad, or iPod touch to iCloud.
WARNING: If you delete the iCloud backup for your iPhone, iPad, or iPod touch, iCloud stops automatically backing up the device.
View and manage iCloud backups on your iPhone, iPad, or iPod touch
1. On your iPhone, iPad, or iPod touch, go to Settings > [your name] > iCloud.
2. Do one of the following:
iOS 16, iPadOS 16, or later: Tap iCloud Backup.
iOS 15, iPadOS 15, or earlier: Tap Manage Storage, then tap Backups.
You see the devices with iCloud Backup turned on.
3. To delete a backup, tap the name of a device, then tap Delete & Turn Off Backup (or Delete Backup).
Thanks for using Apple Support Communities!