update javascript runtime to node20

[theoremoon]

The latest release of this action causes the following error. I guess the oktokit should be updated with the JS runtime at same time.

ERROR fetch is not set. Please pass a fetch implementation as new Octokit({ request: { fetch }}). Learn more at https://github.com/octokit/octokit.js/#fetch-missing

Octokit requires Node 18 or higher, which includes a native fetch API.

[pascalgn]

Yes, good point! Thanks for the PR! 👍