Adapt sandbox script to solidus 4

[mamhoff]

Summary

The automatically generated bin/sandbox script does not work with current Solidus any longer. This upgrades the script to work with (at least) Solidus 4.

It also provides a Rake task to help you obtain this script in existing extensions. From the root of your extension, run

bin/rake extension:upgrade_binstubs

Checklist

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• [x] I have written a thorough PR description.
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• 📖 I have updated the README to account for my changes.

[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:

• bug for bugfixes.
• enhancement for new features and improvements.
• documentation for documentation changes.
• security for security patches.
• removed for feature removals.
• infrastructure for internal changes that should not go in the changelog.

Additionally, the maintainer may also want to add one of the following:

• breaking for breaking changes.
• deprecated for feature deprecations.

Once the correct labels have been set, simply remove the needs changelog label label from this PR so I can merge it.