Update links to main

[helin24]

I think this is what we all need to do to set upstream to main:

git branch -m master main
git fetch upstream
git branch -u upstream/main main
git remote set-head upstream -a

(The default instructions on the repo assume we are cloning this repo but we are all probably working on our own forks)

[jwren]

Thanks for including the directions that we need to follow.

[helin24]

Addresses #8460