Update node version to 20 in GitHub Action

[KyleDolezal]

Overview
Upgrade node version to resolve warnings about out-of-date node version.

Problem
The node version used here is out of date, resulting in a warning ( download-data-json Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20:) when running the action.

Solution
Upgrade to node version 20, as suggested by the warning message. The upgrade has been tested locally by running with node version 20, and index.js continues to behave as it does when running with version 16.

[KyleDolezal]

This is solved by this PR.