Go: Support GOPROXY via the Dependabot proxy

[mbg]

We now support the goproxy_server registry type for Go. This PR updates the LANGUAGE_TO_REGISTRY_TYPE record so that we don't abort getting the credentials for corresponding configurations.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[Copilot]

Pull Request Overview

This PR updates the mapping for the Go language so that Dependabot now correctly uses the "goproxy_server" registry type instead of an empty string.

• Update in src/start-proxy.ts to add the correct registry type for Go
• Update in lib/start-proxy.js to reflect the same change for consistency

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/start-proxy.ts	Added "go: 'goproxy_server'" and removed the duplicated empty entry
lib/start-proxy.js	Updated the Go registry type to "goproxy_server" and removed the empty entry

[aeisenberg]

LGTM.

Do we have an integration test for this?

[mbg]

@aeisenberg We have a test repository for this (I'll send you a link). I am also hoping to look at building a better test framework for the private registry support in the near future, similar to what we currently have for resolve build-environment.