Rename master to main

[hoffmann-stefan]

Now we sould be able to rename the master branches of the following repos:

• https://github.com/ros2-dotnet/ros2_dotnet
• https://github.com/ros2-dotnet/dotnet_cmake_module
• https://github.com/esteve/ament_cmake_export_assemblies

@esteve should ament_cmake_export_assemblies be moved to the ros2-dotnet organization? Seems like both repositories where created on your user, but you only moved dotnet_cmake_module to the ros2-dotnet organization.