[petsc-dev] headsup: switch git default branch from 'master' to 'main'

[Satish Balay]

All,

This is a heads-up, we are to switch the default branch in petsc git
repo from 'master' to 'main'

[Will plan to do the switch on friday the 26th]

We've previously switched 'maint' branch to 'release' before 3.14
release - and this change (to 'main') is the next step in this direction.

Satish