Reference workflows with yml extensions

This updates workflows files to reference resuable workflows with the `.yml`
extension instead of the `.yaml` extension. This is a part of making the name
convention consistent across our repos.

.github/workflows/ci.yml

@@ -30,6 +30,6 @@ jobs:
     if: ${{ github.ref == 'refs/heads/main' }}
     permissions:
       contents: write
-    uses: alphagov/govuk-infrastructure/.github/workflows/publish-rubygem.yaml@main
+    uses: alphagov/govuk-infrastructure/.github/workflows/publish-rubygem.yml@main
     secrets:
       GEM_HOST_API_KEY: ${{ secrets.ALPHAGOV_RUBYGEMS_API_KEY }}