Migrate dotnet/source-build-externals 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 grant me permissions so that I can approve and merge PRs? Thanks

[MichaelSimons]

@dotnet/source-build I will be performing the flat flow migration for this repo. Can you please grant me permissions so that I can approve and merge PRs? Thanks

Granted via product-construction.

[ViktorHofer]

migration-log.json:

{
  "https://github.com/dotnet/arcade - https://github.com/dotnet/source-build-externals": {
    "action": "Disable",
    "id": "350383aa-df0f-4cb0-585b-08d9e6af3ff8"
  },
  "https://github.com/dotnet/source-build-reference-packages - https://github.com/dotnet/source-build-externals": {
    "action": "Disable",
    "id": "5640c8c3-caa9-4077-4856-08da02a24edc"
  },
  "https://github.com/dotnet/roslyn - https://github.com/dotnet/source-build-externals": {
    "action": "Disable",
    "id": "ae4dd5d7-6dd0-40c4-d69f-08da690af6b5"
  },
  "https://github.com/dotnet/source-build-externals - https://github.com/dotnet/sdk": {
    "action": "Disable",
    "id": "6d39c54a-1ca8-4c07-6386-08d9fbbd2f07"
  },
  "https://github.com/dotnet/source-build-externals - https://github.com/dotnet/aspnetcore": {
    "action": "Disable",
    "id": "93759b2e-8cec-490d-6e3a-08da700278bb"
  },
  "https://github.com/dotnet/source-build-externals - https://github.com/dotnet/source-build-reference-packages": {
    "action": "Disable",
    "id": "15ead6e7-8800-4e88-ec3e-08db308e7a41"
  },
  "https://github.com/dotnet/source-build-externals - https://github.com/dotnet/arcade": {
    "action": "Disable",
    "id": "fcfdacb5-90b4-472f-e531-08db30952842"
  },
  "https://github.com/dotnet/source-build-externals - https://github.com/dotnet/runtime": {
    "action": "Disable",
    "id": "b4338da2-58ef-486c-5124-08dbb92a7ffc"
  },
  "https://github.com/dotnet/dotnet - https://github.com/dotnet/source-build-externals": {
    "action": "Create",
    "parameters": {
      "codeflow": true,
      "branch": "main",
      "excludedAssets": ""
    }
  },
  "https://github.com/dotnet/source-build-externals - VMR": {
    "action": "Create",
    "parameters": {
      "codeflow": true,
      "channel": ".NET 10"
    }
  }
}

[ViktorHofer]

Backflow PR: dotnet/source-build-externals#474

Content was incorrect. Microsoft.Build shouldn't get updated. I added it to the backflow subscription's excludeAssets metadata.