Force use of node20 when running the generate-changelog action

[alokedesai]

• Update octokit dependency #124 broke the most recently WarpDev release.
• This is because the version of octokit we depend on requires node 18 or higher: https://github.com/octokit/octokit.js/#fetch-missing
• This PR updates to node 20 directly, GH does not provide a node18 option for runners, after a period of no node updates they decided to directly update to node20. See Support node20 in runs for JavaScript actions actions/runner#2619 (comment)