Ransomware in Lightsail

Hello.

Under Lightsail's shared responsibility model, data and files within the guest OS are the customer's responsibility. AWS does not have the means to decrypt data encrypted by ransomware.
In other words, contacting AWS Support will not allow them to "decrypt" the data; the user must resolve the issue themselves.
If your Lightsail backup is also infected with malware, unfortunately, I think you'll have to recreate it from scratch.