Use defaultSortBy config for loading category products instead of updated_at

[haelbichalex]

Related issues

closes #

Short description and why it's useful

Right now when loading a category page or scrolling to the bottom it will either use the sorting option given in the request or by default it uses updated_at:desc.
In the config there already is an option to set the defaultSortBy. This Pull Requests makes use of it.

Screenshots of visual changes before/after (if there are any)

Which environment this relates to

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• Test version (https://test.storefrontcloud.io) - this is a new feature or improvement for Vue Storefront. I've created branch from develop branch and want to merge it back to develop
• RC version (https://next.storefrontcloud.io) - this is a stabilisation fix for Release Candidate of Vue Storefront. I've created branch from release branch and want to merge it back to release
• Stable version (https://demo.storefrontcloud.io) - this is an important fix for current stable version. I've created branch from hotfix or master branch and want to merge it back to hotfix

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