[cdk-dynamodb-seeder] Update Lambda runtime from Node.js 18

The Lambda custom resource generated by @cloudcomponents/cdk-dynamodb-seeder appears to use the nodejs18.x runtime, which is now unsupported in AWS Lambda.

Could the construct be updated to use a supported runtime, such as nodejs20.x or nodejs22.x?

I’m happy to help with a PR if maintainers are open to it.