Release notes 2 2 8

[jeff-matthews]

This PR is a:

• [x ] New topic
• Content update
• Content fix or rewrite
• Bug fix or improvement

Summary

When this pull request is merged, it will add release notes for the Magento Open Source 2.2.8 and Magento Commerce 2.2.8 releases.

whatsnew
The Magento Open Source 2.2.8 Release Notes and  Magento Commerce 2.2.8 Release Notes provide detailed information about the Magento Open Source 2.2.8 and Magento Commerce 2.2.8 releases.

[magento-cicd2]

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[ghost]

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[dshevtsov]

@jeff-matthews please move whatsnew at the very end of the PR description