Add a "cleanup Renovate" setting

[jamietanna]

There are times when you "offboard" / "uninstall" Renovate from a repository.

A couple of cases:

• Renovate's access (i.e. GitHub App, GitLab PAT) is removed
• The repository had accidentally been onboarded (and may have been activated), but no longer wants to use Renovate
• The repository had started onboarding, but the onboarding PR was closed due to age (onboardingAutoCloseAge)

In these cases, there may be a number of artifacts that are left behind by Renovate, which it does not / cannot remove:

• issues raised by Renovate
• PRs/MRs raised by Renovate
• branches created by Renovate

To handle this in the past, I've hand-crafted a script with the gh CLI and gone through to perform some of this cleanup.

Due to the nature of Renovate's configurability, there might be repo-specific configuration that was used that may mean these are named in a non-standard way, or that aren't as straightforward to detect.

To handle this, we should instead add a global self-hosted configuration option, which will:

• be a self-hosted configuration option
• be environment-variable (or config-file) only (not cli: true)
• be marked as experimental
• run the repo in a new mode (maybe called uninstall)
  • find and close issues raised by Renovate
    • adding a note for why
  • find and close PRs/MRs raised by Renovate
    • adding a note for why
  • find and delete branches created by Renovate
  • default to the authenticated user, but also provide a means to look up a different user
• exit with a non-zero status code
  • to prevent it running multiple times, and to indicate it's not expected to be used often