Split and fix renovate config

[roobre]

When selfhosted, renovate requires a separate config file (ref). This PR splits out self-hosted options to a different file, and points renovate itself to it. Additionally:

• Migrates repo config to json5 (as it allows comments)
• Simplifies the regex looking for grafana-build-tools container image
• Removes gitAuthor, so renovate can rebase PRs created by itself with platformCommit (https://github.com/renovatebot/renovate/discussions/29106)

[mem]

When selfhosted, renovate requires a separate config file (ref). This PR splits out self-hosted options to a different file, and points renovate itself to it. Additionally:

• Migrates repo config to json5 (as it allows comments)

I avoided using json5 because pretty much nothing supports it. There are good reasons for that. If you want comments in json, can I suggest that you use jsonnet? Every json document is a valid jsonnet input.

And it was disappointing to find that renovate explicitly declined supporting YAML as the configuration format. Not that I like YAML, but I admit that it's better than JSON at this kind of thing.

• Removes gitAuthor, so renovate can rebase PRs created by itself with platformCommit (Renovate marks nearly all PRs as "edited" with platformCommit: true renovatebot/renovate#29106)

I copied the configuration from the docs, thinking that was going to provide a simpler path forward. I was so mistaken...

I noticed in the renovate docs that there's some kind of staging / testing functionality. Do you have any experience with that? I would love to avoid the PR / merge / diagnose / PR / ... cycle if possible.

[roobre]

I wish I had. My experience is usually iterating force-pushes to main, as sad as that is. The fact that we're trying to troubleshoot renovate running as an app is pretty unconvenient, because we cannot simply run renovate locally (which is my plan B when forcepushes are not acceptable).

I tried doing that. I failed miserably. I still don't understand why. I would have been happy with a "--dry-run" or "--dont-create-commits" option.

Thanks!

EDIT: Sorry, I clicked the wrong button. I wanted to quote-reply and I clicked "edit". I have no idea how to revert that. 🤷🏽