Run specs on Ruby 2.7, 3.0 and 3.1

[FrancescoAiello01]

Summary

ref #280

Updates CircleCI config to run tests with Ruby 2.7, 3.0 and 3.1. It does not include all permutations of the DB options since they are pretty similar

📝 The specs are currently failing due to the issue with solidus_frontend not being compatible with solidus master (same failure on master)

Checklist

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• [x] I have written a thorough PR description.
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[FrancescoAiello01]

@elia @kennyadsl for visibility. I don't have permission to rerun the specs, but if I add an empty commit, they rerun and pass.

[kennyadsl]

@FrancescoAiello01 CI restarted!

[kennyadsl]

Thanks Francesco!