The dotnet org will rename all default branches to main

We have decided to unify the naming of our default branches to `main`. All new repos will already use this naming. Over the next couple of months, we're going to rename the branches in our existing repos as well. We're using [GitHub's new tooling](https://github.blog/changelog/2021-01-19-support-for-renaming-an-existing-branch/) to make sure branch policies and PRs are automatically updated.

However, this still requires some changes from you as well. This announcement describes the process and what you need to do on your end.

## Process

We'll start with a smaller list of repos and grow from there. Theses are the planned starting repos:

* [dotnet/project-system](https://github.com/dotnet/project-system) ✔
* [dotnet/llvm-project](https://github.com/dotnet/llvm-project) ✔
* [dotnet/icu](https://github.com/dotnet/icu) ✔
* [dotnet/xharness](https://github.com/dotnet/xharness) ✔
* [mono/linker](https://github.com/mono/linker) ✔
* [dotnet/docker-tools](https://github.com/dotnet/docker-tools) ✔
* [dotnet/wcf](https://github.com/dotnet/wcf) ✔
* [dotnet/aspnetcore](https://github.com/dotnet/aspnetcore) ✔
* [dotnet/iot](https://github.com/dotnet/iot) ✔
* [dotnet/winforms](https://github.com/dotnet/winforms) ✔

We're starting with a smaller list to ensure we catch all cases where we need to update corresponding configuration in our Arcade-based tooling (see [this fantastic blog post](https://devblogs.microsoft.com/dotnet/a-deep-dive-into-how-net-builds-and-ships/) on how we build .NET).

Once we have validated our checklist, each repo admin will rename the default branch at their own pace. We expect to complete all repos in the next couple of months.

## What you can expect

*We have a detailed guide you can find at . This is only the essentials.*

We'll add a sticky issue to each repo when we're about to rename the branch. We'll do this a few days before and we'll leave it there for two weeks.

**You don't need to update your PRs**. If you submitted a PR against the old default branch, the GitHub tooling will automatically retarget your PR against the new default branch (main).

For sanity, **you should rename the default branch in your fork as well**. When you go to your fork, GitHub will guide you through this experience:


**You will need to update the repos you cloned to your hard drive**. When you go to the original repository, GitHub will guide you through this experience:


For a discussion, please use https://github.com/dotnet/arcade/issues/6825.