Consider removing Paperclip adapter

[waiting-for-dev]

We're not supporting Rails v5.2 anymore, so all stores using Solidus can use the newer ActiveStorage adapter. However, we might first want to hear about the experience of stores using it. One option can be extracting it to a separate gem and archive it.

[BenMorganIO]

It could cost many applications a lot of dev time to switch the adapter and QA everything. I think it would be best to place it in a separate gem and archive it. Then recommend applications begin using ActiveStorage and make the switch when possible.

[kennyadsl]

I'm experimenting with trying to build a generator that installs the adapter in the host application so that we don't have to create an extension. If a user still uses the Paperclip adapter, we can print a deprecation message asking to run the generator to install the adapter. From that moment, they can keep using the adapter, but they will be responsible for maintaining it. I can push some code soon to show better what I mean, but if you have preliminary feedback on the approach, let me know!

[kennyadsl]

@waiting-for-dev @BenMorganIO I opened a PR with the approach proposed, let me know your thoughts! It's not urgent so take your time.