Adapt to latest Solidus release

[waiting-for-dev]

Summary

Stop trying to pull solidus_frontend v4, which doesn't exist.

Adopt new Solidus default branch. Ref. https://github.com/solidusio/solidus/pull/5042

Checklist

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[mergify[bot]]

It looks like this PR is missing a label to determine the type of change it introduces. The maintainer should add one of the following labels:

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[forkata]

@kennyadsl Any reason we can't merge this so we can get tests running at least against older versions of Solidus and work on getting tests passing against ~master~ main in a separate PR?

[kennyadsl]

I think we can move this forward now @waiting-for-dev, isn't it?

[waiting-for-dev]

Done! Please, let us know if there's any problem.