Updated references master to main

[axel-lauer]

This PR updates all references to "master" e.g. in the documentation and readme files (master branch) to "main" (the new GitHub standard, see https://github.com/github/renaming). See also corresponding PR in ESMValTool: ESMValGroup/ESMValTool#2172.

Closes #1031

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[valeriupredoi]

cheers @axel-lauer let me check and commit if need be the tests conf files 👍

[axel-lauer]

@ESMValGroup/esmvaltool-coreteam are we now theoretically ready to rename master to main and then merge this PR and ESMValGroup/ESMValTool#2172?

[valeriupredoi]

Axel, it's a go from my side, mate! we should probably add a test (GA or Circle) detect any instance of creation of a branch called master after we've switched to main and inspect/delete that, what do you think @bouweandela

[valeriupredoi]

fine by me, cheers much @axel-lauer -> @bouweandela could you give it a look too when you gots some review time pls mate 🍺

[valeriupredoi]

Come on @bouweandela -> maain sounds quite Dutch, surprised you didn't like it 🤣

[bouweandela]

The tests are failing, but it looks like that was solved in #1140, so merging master into this branch will probably solve that?

[valeriupredoi]

on it now @bouweandela - you reckon it'll be ready for merge after? I think it's all OK

[valeriupredoi]

checks have passed, gonna merge, cheers @axel-lauer and @bouweandela 🍺 Shall we delete master once and for good or keep it just in queso