Remove internal gem mirror references; use rubygems.org

Public repo only uses public gems, so drop the commented-out internal
mirror config and reword the publish note to not name internal infra.

Author: dduugg

.buildkite/pipeline.yml

@@ -31,7 +31,6 @@ steps:
           config: .buildkite/docker-compose.yml
 
 # NOTE: gem publishing is intentionally NOT handled here. It currently lives in
-# .github/workflows/cd.yml. If you want releases to move to Buildkite too, add a
-# `wait` + publish step — that step needs the Gusto-internal `gemstash-publish`
-# agent queue and an SSH token-generator plugin, so it only works once this
-# pipeline runs on Gusto Buildkite agents.
+# .github/workflows/cd.yml. Moving releases to Buildkite would require a
+# `wait` + publish step with credentials available only on dedicated agents,
+# so it is left out of this pipeline.

Dockerfile.test

@@ -22,11 +22,6 @@ WORKDIR $APP_HOME
 ENV BUNDLER_VERSION=4.0.7
 RUN gem install bundler --version 4.0.7 --default
 
-# Gusto-internal gem mirror: uncomment ONLY when this pipeline runs on Gusto
-# Buildkite agents that can reach gemstash. Leave commented for public agents,
-# which install straight from rubygems.org.
-# RUN bundle config --local "mirror.https://rubygems.org" "https://gemstash.zp-int.com"
-
 # Install dependencies first so the bundle layer caches across source changes.
 # The gemspec hardcodes its version and globs files with Dir[...], so it loads
 # fine with only the Gemfile and gemspec present.