Algolia PHP Client upgrade to version 4.

[tw2113]

This isn't something we need to get started on ASAP, as version 4 is only in beta, but I'm filing this in preparation for when we do decide to upgrade.

Based on the documentation link below, we will have some breaking changes coming with the new version, so we'll need to handle that at the time.

This also could potentially be a version WPSwA 4.0.x release, due to the client upgrade also being a major one

Resources

https://api-clients-automation.netlify.app/docs/clients/introduction/
https://github.com/algolia/algoliasearch-client-php/tree/next/