Fix deprecated call to `SolidusSupport.solidus_gem_version`
Fix deprecated call to `SolidusSupport.solidus_gem_version`

`SoliduSupport.solidus_gem_version` is deprecated since
`Spree.solidus_gem_version` is available in all the currently
maintained Solidus versions.

solidusio-contrib/solidus_subscriptions,solidusio-contrib/solidus_subscriptions,solidusio-contrib/solidus_subscriptions,solidusio-contrib/solidus_subscriptions