Use Renovate to update external image components

[dotnet/arcade has self-hosted Renovate support now](https://github.com/dotnet/arcade/pull/16504).

We can likely use this to replace most of the code for updating external dependencies in the update-dependencies tool. Candidates are:

- Chisel
- Rocks Toolbox
- MinGit
- Syft
- PowerShell

I have a test version of a renovate pipeline for this repo here: build#2935719. It's failing because it requires https://github.com/dotnet/dotnet-buildtools-prereqs-docker/pull/1608, which is still waiting to be rolled out as far as I can tell.