refactor: keep only the latest release in readme.txt

[Soare-Robert-Daniel]

Summary

• Keep only the latest release in the changelog
• Update the release workflow to replace the changelog only with the latest release.

Will affect visual aspect of the product

NO

Test instructions

You can test the script with

export nextReleaseNotes="Test release notes: Fixed bugs and improved performance." && node bin/update-changelog.js

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes #993

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🎉 This PR is included in version 4.4.15 🎉

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