Rename the master branch to main

[Zeitsperre]

Why?

We have been progressively moving over the trunk names for all of our projects to the new default that was adopted in 2020 (https://www.techrepublic.com/article/github-to-replace-master-with-main-starting-in-october-what-developers-need-to-know/). This repo hasn't moved until now as we didn't want to negatively impact those who've forked our code, but given that the majority of Ouranos projects around xclim have now fully migrated to main, it just makes sense to change it now.

I'd like to see us effect this change in advance of v0.49.0. Thoughts?

[coxipi]

negatively impact those who've forked our code

Any big projects that we should warn as a courtesy? e.g. icclim? I don't know if this change would even affect them.

[aulemahal]

We have 48 forks according to github, but only 7 have seen any push activity in the last 2 years, it goes down to 3 for the last year (including one bot). I think we're safe!

[Zeitsperre]

negatively impact those who've forked our code

Any big projects that we should warn as a courtesy? e.g. icclim? I don't know if this change would even affect them.

I don't think so. icclim based on our stable releases, so no problem there. Right, @bzah?

[bzah]

Yep, no issues for icclim :)