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/release.yml

@@ -2,7 +2,7 @@ name: Release
 on:
   push:
     branches:
-      - master
+      - main
     paths:
       - 'desktop/package.json'
 
@@ -38,7 +38,7 @@ jobs:
           tag_name: ${{ steps.tag-version-commit.outputs.tag }}
           release_name: ${{ steps.tag-version-commit.outputs.tag }}
           body: |
-            See https://github.com/facebook/flipper/blob/master/desktop/static/CHANGELOG.md
+            See https://github.com/facebook/flipper/blob/main/desktop/static/CHANGELOG.md
             for full notes.
           draft: false
           prerelease: false