Rename default branch to `main`

[lukaspie]

I think by now it is relatively standard to use main instead of master and we also do that in all other packages, so we should probably change the default branch name at some point.

Will require changes in the following places:

• pynxtools-mpes pytest CI
• pynxtools-igor pytest CI
• pynxtools-spm pytest CI
• nomad-distro test-oasis

cc @mkuehbach