Rename master to main

[lbdreyer]

See https://github.com/github/renaming

As we have already started making this change to other repos in the SciTools (see iris discussion, nc-time-axis)

If there are no objections, I'd like to push ahead by the end of next week

[lbdreyer]

@pp-mo has kindly renamed the branch, so this can be closed.

[trexfeathers]

Note that README.md has several badges that still point to master.