CI: Use Solidus' reusable GH workflow for testing

[tvdeyen]

Summary

Needs solidusio/test-solidus-extension#3

Instead of setting up all services and the matrix
in this extension we use the new reusable workflow
that handles this for us.

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• I agree that my PR will be published under the same license as Solidus.
• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have localized any and all user-facing strings that I added to the source code.
• I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.

[mamhoff]

Have a look at the Gemfile as well - I think we're not adding the necessary ~> before the Rails version.

[mamhoff]

Sweet!