Update README: update Frontend URL

[D-system]

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Summary

The frontend has been removed in #4497 and moved to https://github.com/solidusio/solidus_frontend Removing the frontend from the list of the default included gems.

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• [x] I have written a thorough PR description.
• [x] I have kept my commits small and atomic.
• [x] I have used clear, explanatory commit messages.

The following are not always needed (~cross them out~ if they are not):

• [ ] ~I have added automated tests to cover my changes.~
• [ ] ~I have attached screenshots to demo visual changes.~
• [ ] ~I have opened a PR to update the guides.~
• [x] I have updated the readme to account for my changes.

[waiting-for-dev]

Hi @D-system, and thanks for your collaboration. Although the code has been removed from this repo, solidus_frontend is still a part of the solidus meta-gem, so what the README says it's still true. A little below, it goes on:

While solidus_frontend is still present in the meta-gem, it'll be removed in Solidus v4.0. For new stores, we recommend using solidus_starter_frontend.

Let's put your PR on hold for Solidus v4.0, but we'll need to update the previous paragraph.

[D-system]

Hi @waiting-for-dev Thank you for pointing out it was required and listed in the gemspec. I update the PR to point to the new repository and updated the title of this PR.

[D-system]

Updated in another PR. Closing it.