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

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