sg: switch from syntect_server to syntax-highlighter in dev
Created by: chrismwendt
Previously, in local development, we were still using the old https://hub.docker.com/r/sourcegraph/syntect_server (from before migrating https://github.com/sourcegraph/syntect_server to https://github.com/sourcegraph/sourcegraph).
After this change, we will use https://hub.docker.com/r/sourcegraph/syntax-highlighter in local development.