dev/Caddyfile: use 127.0.0.1 instead of localhost

Created by: bobheadxi

This aligns with what Sourcegraph services listen on in dev with INSECURE_DEV mode.

I noticed Rob getting indismissible popups with Caddy - this didn't quite solve that, but it did solve his browser treating sourcegraph.test as insecure, and seems to be a reasonable change regardless to align with other services. We also have an anti-localhost linter with additional reasoning.

Also see: https://github.com/sourcegraph/sourcegraph/pull/34714

Test plan

sg start web-standalone