[meta] Support neutral naming for EUI default branch

[thompsongl]

As part of a larger neutral naming effort at Elastic, we want to move from using master to main as the default branch name in GitHub. This involves:

• Update references to master in code (Update references to the default branch name #5344)
  • Changelog
  • Comments
  • Links
  • Wiki
  • Scripts
  • CI
• Rename master to main in the GitHub UI - 8AM PT (UTC−07:00) November 4

Reference updates will be completed in a branch in advance, and will be merged immediately upon name change completion.

Renaming a branch will:

• Re-target any open pull requests
• Update any draft releases based on the branch
• Move any branch protection rules that explicitly reference the old name
• Update the branch used to build GitHub Pages, if applicable
• Show a notice to repository contributors, maintainers, and admins on the repository homepage with instructions to update local copies of the repository
• Show a notice to contributors who git push to the old branch
• Redirect web requests for the old branch name to the new branch name
• Return a "Moved Permanently" response in API requests for the old branch name

Anyone with a local clone of the repository will need to update their clone. The following commands assume a remote name of upstream for the Elastic organization's repository. If you use something different, be sure to replace the remote name in the following commands.

$ git branch -m master main
$ git fetch upstream
$ git branch -u upstream/main main
$ git remote set-head upstream -a

[thompsongl]

Rename complete. If any problems arise, they can be addressed in a dedicated issue.