[Feature]: Rename branch `master` to `main`

[jay-tau]

What feature do you want?

Rename the master branch to main

[Sumit-ai-dev]

Hey! I took a look at this and went ahead and bumped the default branch to main.

I just renamed it locally and verified all the hardcoded image links in the README (for the Darshan graphs) are pointing to main now so they don't break.

The new branch is pushed up in this PR. Once it's merged, an admin will just need to quickly hop into the repo settings and swap the default branch over to main before we delete master.

Let me know if this looks good!