fix(W-17265927): SSL issues with heroku logs

[justinwilaby]

This PR replaces @heroku/eventsource with a modern fetch implementation.

Why are we doing this?

1. Multiple reports of SSL errors have been seen. logplex urls sometimes use redirects on client machines with HTTP_PROXY. Native fetch handles this for us.
2. The EventSource API is now a widely available web standard but it is not available in Node. The @heroku/eventsource library was meant as a polyfill for Node however, Node is not adopting this API. Instead, fetch is commonly used for Server Side Events in node
3. @heroku/eventsource is very old and is no longer active in development (security fixes only)

Testing

1. run bin/run logs -a <APP_NAME> --tail

[zwhitfield3]

Ran great! You might have to restart that one integration test, but otherwise, looks good to go 👍