Default branch change affecting `remotes::install_github`

[dcollins15]

As part of a larger effort to update our group's CI/CD workflows, we will be deprecating the master and develop branches in favor of cran and main respectively. For now, we will leave the old branches in place to give folks a chance to transfer their workflows over.

We have also updated the default branch for this repository from master to main. This change will affect users who install the package using remotes::install_github without setting the ref parameter. To install the latest CRAN release from GitHub users should now call:

remotes::install_github("satijalab/seurat", ref = "cran")