Use GitHub Actions for Renovate

[blarghmatey]

Description/Context

This repository has two steps that need to be executed as part of making updates to the dependencies which are incompatible with the GitHub App version of Renovate. In particular, every PR needs to have a corresponding changelog entry in order to be able to merge, and when updating the dependencies the Pants lockfile also needs to be updated.

Renovate, when run in a self-hosted manner, has a configuration option of executing postUpgradeTasks as part of the upgrade step. Renovate also has a GitHub Action available as a means of running it on a repository, which constitutes a mode of self-hosting. To that end, we need to migrate the usage of Renovate in this repository to use the GitHub Action, and update the Renovate configuration to execute the postUpgradeTasks necessary to regenerate the Pants lockfile and create the required changelog entries for the libraries.

Plan/Design

• Create a GitHub Actions workflow file for executing Renovate on a scheduled basis (at least daily)
• Configure Renovate to run the postUpgradeTasks needed to generate the proper changelog entries
• Configure Renovate to regenerate the Pants lockfiles
• Remove this application from the GitHub Apps version of Renovate

[pdpinch]

Willl all our monorepos need this approach, e.g. https://github.com/mitodl/open-edx-plugins ?

[blarghmatey]

No, this is particular to the workflow of this repository because of the requirement to have changelog entries for every PR, necessitating a custom step to execute as part of the PR creation.