Updates to default branch names

[coleramos425]

We will be renaming the Omniperf branches to align with the AMD standard from Dev-Ops. main--> amd-mainline, dev --> amd-staging, then we'll make amd-staging the default branch.

If you are working on these branches on your local clone, then you will want to run :

git branch -m OLD-BRANCH-NAME NEW-BRANCH-NAME
git fetch origin
git branch -u origin/NEW-BRANCH-NAME NEW-BRANCH-NAME
git remote set-head origin -a

(https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-branches-in-your-repository/renaming-a-branch)

Thank you!