Revert "CI: use latest rubygems (#4681)"

This reverts commit a5cccf5.

This fix is only required for Ruby 3.0.
As required ruby version is bumped up to 3.2, no need to apply anymore.

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

.github/workflows/test.yml

@@ -35,7 +35,6 @@ jobs:
         uses: ruby/setup-ruby@401c19e14f474b54450cd3905bb8b86e2c8509cf # v1.204.0
         with:
           ruby-version: ${{ matrix.ruby-version }}
-          rubygems: latest
       - name: Install addons
         if: ${{ matrix.os == 'ubuntu-latest' }}
         run: sudo apt-get install libgmp3-dev libcap-ng-dev