Rename default "master" Branch >> "main"

[MarkusTiede]

why: https://github.com/github/renaming#renaming-the-default-branch-from-master

Repos & issues we have so far:

• Master >> Main repository-template-java#15

[MarkusTiede]

Done for the open-source repo itself via: https://github.com/github/renaming#renaming-existing-branches

[MarkusTiede]

Done for : https://github.com/baloise/digital-signature/branches with @Tiliavir onboard.

[MarkusTiede]

Done again for: https://github.com/baloise/repository-template-nosrc

[MarkusTiede]

Create issues for outstanding repos.

[MarkusTiede]

baloise/org#40

[MarkusTiede]

Rename master branch to main https://github.com/baloise/proxy/issues/12

[MarkusTiede]

BOC-Group/developer-examples#13