Migrate dotnet/source-build-reference-packages onto the flat flow

[premun]

• Run the migration tool in dry run (to backup the subscription changes)

  dotnet run -- migrate --output migration-log.json [repo]
  

• Share the contents of the migration log in this issue
• Verify the migration log - basic sanity check
• Run the migration tool in full mode

  dotnet run -- migrate --perform-updates [repo]
  

• Go to https://maestro.dot.net/subscriptions and perform a quick sanity check on the subscriptions of this repo
• Trigger the backflow subscription
• Validate the opened backflow PR
  • Updates should include mostly be version files (like we did an old dependency flow), eng/common files
  • Validate that Arcade major version has not been changed
  • Validate that there's no missing feeds in the PR build (push into the PR if there are)
• Look through other active dependency flow PRs in the repo and close & comment on them
  • Let them know that there will be conflicts if they choose to merge it still
  • Link the guide

[ViktorHofer]

@dotnet/source-build I will be performing the flat flow migration for this repo. Can you please double check that I have permissions to approve and merge PRs? Thanks