Spree Digital v4.0.0

Rails 6.0 support
Spree 4.0 compability
Spree 3.5 and lower support is dropped
Replaces paperclip support with active storage
Updated rspec-rails to 4.0.0.beta2
Updated pg from 0.18 to current
Updated mysql2 from 0.5.1 to current
Added selenium-webdriver development dependency
For change upgrade path from paperclip to ActiveStorage use https://github.com/thoughtbot/paperclip/blob/master/MIGRATING.md#moving-files-on-a-remote-host-s3-azure-storage-gcs-etc

Provide feedback