Rename default branch references to main (#2689)

Summary:
I really hope I hit everything and didn't break anything that wasn't
referring to branch names.

Pull Request resolved: https://github.com/facebook/flipper/pull/2689

Test Plan:
_eyes

CI

|[Site Preview: flipper](https://our.intern.facebook.com/intern/staticdocs/eph/D30305789/V2/flipper/)

Reviewed By: timur-valiev

Differential Revision: D30305789

Pulled By: passy

fbshipit-source-id: 5daaa09250bb96bb50be679fc01dae86c666eb73

.github/workflows/publish-pods.yml

@@ -131,7 +131,7 @@ jobs:
             - Make sure that the Podfile.lock contains latest FlipperKit and Flipper pod versions.
             - Also make sure that all the dependencies are updated to the latest one.
             - This is auto-generated by [create-pull-request](https://github.com/peter-evans/create-pull-request)
-          base: "master"
+          base: "main"
           branch-suffix: short-commit-hash
           labels: automated pr
         env: