Migration to main completed

As [previously announced](https://github.com/dotnet/announcements/issues/172), we have standardized the branch naming and use `main` as the default branch for all new repos moving forward. We have just completed the migration of all existing repos to `main` in the `dotnet` and `aspnet` organizations.


## What about non-Microsoft owned repos?

There are other repos in the `dotnet` organization that aren't owned by Microsoft. It's up to the project maintainers to decide whether or not they want to rename their default branch as well. We decided that there is a value in having a unified naming convention across all our repos as opposed to having a split like "new repos use `main`, existing repos use what they currently have". But renaming the default branch is work and there is a reason it took as couple of months to complete the migration to `main`.

In case you're curious, [here are the steps we followed](https://aka.ms/m2m-dotnet).

## What do I need to do?

**You don't need to update your PRs**. If you submitted a PR against the old default branch, the GitHub tooling has automatically retargeted 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.