Refactor weekly update script to run as a `github-script` action

[dhruvkb]

Description

The weekly update script last_week_tonight.mjs runs as a standalone Node.js script. It is the last remaining standalone script in the JS based automations. Converting it to a actions/github-script based job would bring the following improvements:

• consistency with other jobs like issue and PR automations
• uniformity of code structure and documentation
• identical guides for testing these scripts locally
• possibility to unit test all workflow jobs using common patterns #1916
• ability to convert @octokit/rest to a dev-dependency

This is closely related to #3445 as they both deal with improvements to the same workflow.