chore: use changelog for GitHub release notes

[marandaneto]

💡 Motivation and Context

GitHub releases should use the SDK changelog entry as the release body instead of auto-generated release notes, so published release notes match the changelog markdown.

💚 How did you test it?

• Ran git diff --check.
• Reviewed the release workflow diff against other SDK release workflows that read CHANGELOG.md before creating GitHub releases.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file