[Backport] Fixed wrong proxing in the inventory observer

[VitaliyBoyko]

Original Pull Request

#21731

Description (*)

FIxed the wrong proxy resourceStock argument for the \Magento\CatalogInventory\Observer\UpdateItemsStockUponConfigChangeObserver.

Module di.xml
<argument name="resourceStock" xsi:type="object">Magento\CatalogInventory\Model\ResourceModel\Stock\Proxy</argument>
changed to
<argument name="resourceStockItem" xsi:type="object">Magento\CatalogInventory\Model\ResourceModel\Stock\Item\Proxy</argument>

Fixed Issues (if relevant)

These changes are needed for
magento/graphql-ce#167: Add support for '@magentoConfigFixture' annotation on GraphQL API-functional tests
PR: magento/graphql-ce#351

Manual testing scenarios (*)

No testing scenarios.

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 @VitaliyBoyko. Thank you for your contribution
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[magento-engcom-team]

Hi @dmytro-ch, thank you for the review.
ENGCOM-4514 has been created to process this Pull Request

[soleksii]

Not for manual testing

[ghost]

Hi @VitaliyBoyko, thank you for your contribution!
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