Upgrade to the latest version of solidus

[elia]

The current solidus is 2.11 but 3.2 is already out and 3.3 is around the corner.

This should be upgraded in one of two ways:

1. We consider this an existing app and we upgrade it as usual
2. We try to rebuild the app from scratch and reapply the changes that were required for the demo behavior, this has the advantage that it would also use the latest version of the starter frontend

The second approach seems more complex, but maybe we can do the first and still move a little in the direction of the second, so that next time we can do the jump.

[chrean]

@aldesantis to provide some additional context. On hold for now.