Update sandbox.tt Solidus install generator usage

[RyanofWoods]

Summary

The --with-authentication flag is deprecated in favour for the --authentication flag that supports the following options: devise, existing, custom, none.

Checklist

• I have structured the commits for clarity and conciseness.
• I have added relevant automated tests for this change.

[mergify]

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.

[elia]

I'm a bit conflicted about this, as the new option is only available on solidus master, and the old one is deprecated but still available, was it only a matter of solving the deprecation or bumped into some other issue?

[RyanofWoods]

I understand @elia, is it because we want to have old Solidus version support?

It was just to get rid of the deprecation.

[elia]

I understand @elia, is it because we want to have old Solidus version support?

Yes, so far solidus_dev_support was designed to help in making extensions compatible with all currently supported versions of Solidus (i.e. not EOL). So you could export SOLIDUS_BRANCH=v3.0 and bin/setup and then recreate the sandbox using that version to try the extension on a real app with the version of solidus you want.

Hope this makes sense.