Build: Switch to standard rubygems.org

The RubyGems registry offered by GitHub Packages is not public,
it requires installers to specify authentication tokens even for
"public" packages, which is imho unreasonably difficult for something
this trivial.

Switch to the public registry instead.

.github/workflows/gem.yaml

@@ -7,24 +7,19 @@ on:
 jobs:
   gem-push:
     runs-on: ubuntu-latest
-    permissions:
-      # https://docs.github.com/en/actions/reference/authentication-in-a-workflow
-      packages: write
     steps:
     - uses: actions/checkout@v2
     - name: Publish Gem package
       env:
-        GITHUB_TOKEN: "${{ secrets.GITHUB_TOKEN }}"
+        GEM_HOST_API_KEY: "${{ secrets.RUBYGEMS_API_PUSH }}"
       # Inspired by https://github.com/bodyshopbidsdotcom/gh-action-publish-gem-on-tag/blob/1.0.1/entrypoint.sh
+      # Docs: https://guides.rubygems.org/command-reference/#gem-push
       run: |
         GIT_VERSION=$(echo "$GITHUB_REF" | cut -d'/' -f 3-)
         SPEC_VERSION=$(ruby -e 'require "rubygems"; gemspec = Dir.entries(".").find { |file| file =~ /.*\.gemspec/ }; spec = Gem::Specification::load(gemspec); puts spec.version')
         if [[ "$GIT_VERSION" != "v${SPEC_VERSION}" ]]; then
           echo "Ignoring, git tag version differs from gem spec version."
           exit 2;
         fi
-        mkdir -p ~/.gem
-        echo ":github: Bearer ${GITHUB_TOKEN}" > ~/.gem/credentials
-        chmod 600 ~/.gem/credentials
         gem build *.gemspec
-        gem push --key github --host "https://rubygems.pkg.github.com/${{ github.repository_owner }}" *.gem
+        gem push *.gem