Clean up multiple set-cookie header handling

## Prerequisites

* [x] I searched to see if the issue already exists.
* [x] Are you an Up Pro subscriber?

## Description

Amazon added support for multiple instances of headers, so we can clean up Set-Cookie support. Rather than doing the goofy `CaSE-staggering` stuff, we can just use `multiValueHeaders`: https://aws.amazon.com/about-aws/whats-new/2018/10/amazon-api-gateway-adds-support-for-multi-parameters/
https://docs.aws.amazon.com/apigateway/latest/developerguide/set-up-lambda-proxy-integrations.html#api-gateway-simple-proxy-for-lambda-output-format

Relevant: #673 and #675
https://github.com/apex/up/blob/5f16b7f99d6c5bd23289e8bf1927a5cce32e3475/internal/proxy/response.go#L63