Replace obsolete 'Generate Token' step, thanks to improved Scala Steward GitHub Action support

[rtyley]

(initially raised as #44)

scala-steward-org/scala-steward-action#428 updated Scala Steward's GitHub Action to have its own support for GitHub App authentication, meaning we can remove the additional 'Generate token' step from our GitHub Action workflow, with these benefits:

• No longer need to use the tibdex/github-app-token - one fewer action we need to trust!
• Running the Scala Steward action on a repo no longer requires adding the repo to REPOSITORIES.md - only granting the installation access to the repo is required:
  
  • I've reduced REPOSITORIES.md to just the repos that have custom configuration (eg the :dependency-updates suffix in guardian/amigo:dependency-updates).

Testing

I've test-run the workflow against this branch, and it succeeded:

https://github.com/guardian/scala-steward-public-repos/actions/runs/5154533830

[rtyley]

One surprise here is that with moving from tibdex/github-app-token to scala-steward-org/scala-steward-action for App authentication, we now need to also supply the github-app-installation-id, where previously we didn't.

In the Scala Steward action, both 'app' & 'installation' tokens seem to get used - so far as I can see, the 'app' is used for performing actions as the bot, while 'installation' is used for in the Scala Steward workspace - I think for cloning the required GitHub repos.

In our case, because we own both the app and the installation, the distinction probably doesn't help us very much, but I imagine it's useful if you're an organisation like VirtusLab, who run Scala Steward on other organisations repositories as a service.