Change default branch name to `main`

[joemull]

We have discussed changing our 'master' branch to 'main' at various points, to meet our shared goal of fostering an open and welcoming environment for everyone as well as upholding Birkbeck's foundational commitment to equity and diversity.

Changing the default branch name can present a few logistical hurdles with branches in development, so it is best to find the right moment between releases, in the same way as we might do with integrating the Python code style enforcer Black. So it might be worth tackling these two issues at the same time, after the release of Janeway 1.5.1.

[ajrbyers]

We should go over all the repos and sort them all. Short term it might cause some pain but it’ll work out.

[joemull]

We should go over all the repos and sort them all. Short term it might cause some pain but it’ll work out.

Yes, I figured once the main repo changes the others should change at the same time.

[joemull]

I am going to wait for typesetting PRs to clear up and then do this.