Rename develop branch to main, drop master branch?

[giovannipizzi]

This came from some old recommendation, but in the end now GitHub recommends main for the main branch, and the master was to point to the latest stable release, but honestly it's hard to keep it synced and it's easier to just tell people to check out the correct git tag (or download a given version).

So I would suggest this change and we only have 1 main branch

Pinging @mostofi @jryates @JeromeCCP9 for feedback

[giovannipizzi]

Note: if we do it, we need to also check potential broken links if we have direct links to a file on GitHub (e.g. on the People website there is a link to the contributors that points to the AUTHORS.md on GitHub with link https://github.com/wannier-developers/wannier90/blob/develop/AUTHORS.md that will thus change once we rename