Change error message about low stock

[vkublytskyi]

Change error message about low stock from We don\'t have as many "{product name}" as you requested. to The requested qty is not available.

Why this change is required?

MSI project introduces a new mechanism of inventory management for Magento 2. Now MSI and Magento Open Source have different error messages that may introduce difficulties for switching from current inventory management to MSI and complicate test suite.

Why core is fixed instead of MSI?

MSI implementation strongly follows SOLID principles. At the point where an error message is created no information about a product available. Any possible fixes will break encapsulation, introduce undesirable coupling and lead to performance degradation.

Change of messages in core has no cost and no significant impact on user experience

[magento-engcom-team]

Hi @vkublytskyi. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
• @magento-engcom-team give me {$VERSION} instance - deploy vanilla Magento instance

For more details, please, review the Magento Contributor Assistant documentation

[magento-engcom-team]

Hi @vkublytskyi. Thank you for your contribution.
We will aim to release these changes as part of 2.3.0.
Please check the release notes for final confirmation.