Switch default branch to main

We want to switch the default branch name from master to main for all gridscale Open Source projects including this repository.

This involves:
- Switching the default branch name here on GitHub
- Updating all references of the master branch name in GH Actions and other CI/CD pipelines and scripts
- Giving contributors a bit of info on how to update their remotes in their local copies of the repo

Would be nice to do this coordinated and in "one go" so that contributors are not surprised by inconsistent naming or repositories not having switched yet. If you want to help out here contact @bkircher

首先，盘点 GitHub Actions、其他 CI/CD 流水线、脚本和贡献者指南中对 master branch 的引用。检查仓库的 GitHub default-branch 设置，并确定必须同步更改的其他 gridscale Open Source 项目。完成的标准是协同切换已完成，并且贡献者拥有更新本地 remotes 的说明。

由索引模型根据 Issue 内容生成。