Update GitHub default branch name for version_dev argument

[santisoler]

Description of the desired feature

After GitHub changed the default branch name from master to main, we should update Pooch docstrings to include main as one of the best choices for the version_dev argument, like in:

https://github.com/fatiando/pooch/blob/1f60c6ffe143a233bf15b5d1d1ac5acc648504ba/pooch/core.py#L282-L285

We could deprecate plan to change the default value for version_dev to main after Pooch v2.0.0 and add a Deprecation Warning meanwhile.

Are you willing to help implement and maintain this feature? Yes, but I would let anyone to tackle it if they want!

[santisoler]

Since "master" is the default value for the version_dev parameter in pooch.create, changing it to "main" would break backward compatibility. I think this issue should be targeted for next major release of Pooch.