Fix CI

[waiting-for-dev]

Summary

Only test old subscribers compatibility when legacy system is present

The legacy event system is not available since v4.0, so constants won't be available for testing.

Update CI configuration

Run different Solidus versions on different jobs for better error handling.

We're now explicitly providing the ruby version to the executor while using a matrix configuration for better extensibility.- Only test old subscribers compatibility when legacy system is present

Checklist

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