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

react-native/react-native-flipper/android/build.gradle

@@ -98,7 +98,7 @@ def configureReactNativePom(def pom) {
         licenses {
             license {
                 name packageJson.license
-                url packageJson.repository.baseUrl + '/blob/master/' + packageJson.licenseFilename
+                url packageJson.repository.baseUrl + '/blob/main/' + packageJson.licenseFilename
                 distribution 'repo'
             }
         }