Rename zoekt external gateway container

What does this MR do and why?

This MR:

• renames the container for the external gateway from zoekt-gateway to zoekt-external-gateway.
• changes comments that reference Main Svc to External Gateway Svc in _helpers.tpl

The change is to help differentiate the gateway containers in logs. It will be easier to have two distinct matchers

How to set up and validate locally

1. check out main branch
2. install the chart: helm install gitlab-zoekt .
3. run the integration script, but skip the install step: SKIP_INSTALL=1 ./spec/scripts/integration.sh
4. check out this branch (tchu-rename-external-gateway)
5. run the integration script, but skip the install step: SKIP_INSTALL=1 ./spec/scripts/integration.sh