Fix wrong return type in StockRegistryInterface::getLowStockItems() API

[vasilii-b]

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 @vasilii-b. Thank you for your contribution
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[rogyar]

Hi @vasilii-b. Thank you for your commitment. According to the latest contributor guide (https://devdocs.magento.com/guides/v2.2/contributor-guide/contributing.html) code cleanup should be performed in 2.3-develop branch. Please, create PR to the mentioned branch.
Thank you.

[vasilii-b]

Hi @rogyar,
Thank you for update. PR for 2.3 was created - #17562