[Backport] Fix wrong return type in StockRegistryInterface API

[lewisvoncken]

Original Pull Request

#17562

Description

Full description in #15085.

Fixes wrong return type of StockRegistryInterface :: getLowStockItems() when using REST API /V1/stockItems/lowStock/

Fixed Issues (if relevant)

1. StockRegistryInterface :: getLowStockItems() returns StockStatusCollection instead of StockItemCollection #15085

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @josefbehr, thank you for the review.
ENGCOM-3133 has been created to process this Pull Request

[magento-engcom-team]

Hi @lewisvoncken. Thank you for your contribution.
We will aim to release these changes as part of 2.2.8.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.