Rename master branch

Do we need to rename our existing repositories master branches or can they be left as is?

The choice to rename or not a branch from ‘master’ is up to you.

In some cases (especially GitHub actions) things will break if you perform a rename. See the documentation:
https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-branches-in-your-repository/renaming-a-branch#about-renaming-branches

But, in general, it’s up to you.