Remove reference to dead link about GH actions (#6527)

Fixes #6526

Author: parlough

src/content/tools/testing.md

@@ -120,15 +120,10 @@ and run its tests after every commit. Two CI services for GitHub are
 
 Learn more about GitHub Actions:
 
-* [Testing Dart packages with GitHub Actions][gha-article]
-  demonstrates a simple workflow for using GitHub Actions to test
-  a Flutter app or Dart package.
 * Many packages provided by the Dart team use GitHub Actions.
   For an example, see
-  [`test-package.yml`][markdown-ci] in the markdown package's repo.
+  [`markdown.yaml`][markdown-ci] in the markdown package's repo.
   To see how that repo migrated from Travis CI to GitHub Actions,
   look at [PR #353]({{site.repo.dart.org}}/markdown/pull/353).
 
-[gha-article]: https://poetryincode.dev/testing-dart-packages-with-github-actions
-[markdown-ci]: {{site.repo.dart.org}}/markdown/blob/master/.github/workflows/test-package.yml
-
+[markdown-ci]: {{site.repo.dart.org}}/tools/blob/main/.github/workflows/markdown.yaml