Renaming `master` branch to `main`

[zormit]

It's maybe a bit cosmetics, but what would you think about renaming the branch according to more neutral language?

[crosspolar]

Good idea, especially since it's the new default of git. We should check if "master" is hardcoded somewhere in the CI and the infrastructure repo.

[zormit]

Steps (to be extended)

• start with creating an unqualified main branch (s.t. the CI can reference to it and nothing breaks)
• PR: change hard-coded master to main e.g. in .github/workflows/docker-build.yml, .github/workflows/codeql-analysis.yml, CONTRIBUTING.md - did I miss something?
• check infrastructure repo
• configure main to be the new qualified default branch on Github