chore: add Node.js Agent v12.8.0 Release Notes

[newrelic-node-agent-team]

This is an automated PR generated when the Node.js agent is released. Please merge as soon as possible.

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[github-actions]

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[github-actions]

[!NOTE]

Our Vale automated writing feedback tool has suggestions for this PR. You can view these suggestions in the Files changed tab. If you're not a New Relic docs writer, we'll review any comments and you don't need to take any action (though feel free to make edits if you like!).

[github-actions]

[!NOTE]

Our Vale automated writing feedback tool has suggestions for this PR. You can view these suggestions in the Files changed tab. If you're not a New Relic docs writer, we'll review any comments and you don't need to take any action (though feel free to make edits if you like!).

[adutta-newrelic]

Looks good to me