Stop doing unwanted NuGet.config "cleanup"

- [ ] This issue is blocking
- [x] This issue is causing unreasonable pain
Where "unreasonable" rhymes w/ "unnecessary".

Maestro++ / `darc` dependency updates e.g. for dotnet/arcade ➡️ dotnet/aspnetcore clear out unused `packageSources` and `disabledPackageSources` as part of the public PRs. If the affected branch is in servicing e.g. release/6.0, this prevents normal processing in the [`internal-code-merge` pipeline](https://dev.azure.com/dnceng/internal/_build?definitionId=240). See [build 2045204](https://dev.azure.com/dnceng/internal/_build/results?buildId=2045204&view=results) for an example where NuGet.config had merge conflicts due to a public Arcade update PR.

I'd like to see us change Maestro++ / `darc` to _not_ clear out "unused" feeds and disabled package sources when taking Arcade dependency updates. A manual change or something automatically included in our regular "merge from internal" PRs (what @vseanreesermsft does) would be much less likely to cause problems.

1. what those updates clear is for repos that aren't named "arcade"
2. making those changes anytime other than when merging from internal tends to cause hurt.

Note: It's generally fine for (say) a dotnet/runtime ➡️ dotnet/aspnetcore dependency update PR to clear out unused feeds **for dotnet-runtime or dotnet/runtime** because the subscription means dependencies are flowing in public.

### Release Note Category
- [ ] Feature changes/additions
- [ ] Bug fixes
- [ ] Internal Infrastructure Improvements
### Release Note Description