💚 Fix CI failing on unknown Ruby 3.4.7
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ruby/setup-ruby v1.238.0 predates Ruby 3.4.7, so every CI job failed at Set up Ruby. Bump to v1.321.0.
ruby/setup-ruby v1.238.0 predates Ruby 3.4.7, so every CI job failed at Set up Ruby. Bump to v1.321.0.