What is Password for Backup, whilst restoring onto my New iPhone 13?

Only you would know that. You and you alone set that passcode to encrypt the backups. The password is not stored anywhere else and nobody but you can know it.

If you no longer remember the passcode, then you will need to turn off encryption and reset the passcode as explained in the support document you posted from, and create a new unencrypted backup if possible you can use for your new iPhone.

The encrypted backups cannot be accessed without the passcode used to encrypt them.