Dependabot on steroids: Update DefaultVersions.props

[ViktorHofer]

https://github.com/dotnet/arcade/blob/d83761aff116d58009415d85889625c4d38d809e/src/Microsoft.DotNet.Arcade.Sdk/tools/DefaultVersions.props

It would be good to update the versions in DefaultVersions.props as many of those are stale or manually updated, i.e. #16985.

We already have dependabot enabled in this repository for in-repo dependencies. That infrastructure isn't able to handle these sdk provided properties though. An agentic workflow that mimics dependabot could be set-up to keep those up-to-date.

[ViktorHofer]

Requirements:

1. Need to be able to parse the the package name from the version property. If not possible, skip the property.
2. Look at nuget.org + the configured feeds from the NuGet.config for a newer version. Explicitly also look at nuget.org as many of these packages are only syned on demand.
3. If the newer package version only exists on nuget.org but not on the dotnet-public feed, add a table entry to the PR requesting a human to add the new version by following this doc: https://github.com/dotnet/arcade/blob/main/Documentation/MirroringPackages.md. Still make the change, the PR will fail until the new package versions are available on the custom feeds. That's fine.
4. If some of these properties are also defined in https://github.com/dotnet/arcade/blob/main/Directory.Packages.props, update them there as well so that the arcade repository build uses the up-to-date version.
5. If a section has a comment with special instructions, consider them as well. This currently applies to the xunit packages which link to https://github.com/dotnet/arcade/blob/main/Documentation/update-xunit.md.

I think a github agentic workflow could be a great way to implement this, running on a schedule similar to dependabot.