Update link branches to main (#146558)

- Update CS and googlesource.com link branches
- Update GitHub /blob/ and /tree/ links

Tested links manually and fixes a few broken or deprecated links

Added a test that validates that `master` isn't used, except for
specified repos.

Part of https://github.com/flutter/flutter/issues/121564

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[test-exempt]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Data Driven Fixes]:
https://github.com/flutter/flutter/wiki/Data-driven-Fixes

README.md

@@ -25,7 +25,7 @@ open source.
 * [Install Flutter](https://flutter.dev/get-started/)
 * [Flutter documentation](https://docs.flutter.dev/)
 * [Development wiki](https://github.com/flutter/flutter/wiki)
-* [Contributing to Flutter](https://github.com/flutter/flutter/blob/master/CONTRIBUTING.md)
+* [Contributing to Flutter](https://github.com/flutter/flutter/blob/main/CONTRIBUTING.md)
 
 For announcements about new releases, follow the
 [flutter-announce@googlegroups.com](https://groups.google.com/forum/#!forum/flutter-announce)
@@ -121,5 +121,5 @@ Information on how to get started can be found in our
 [macOS FFI]: https://docs.flutter.dev/development/platform-integration/macos/c-interop
 [Windows FFI]: https://docs.flutter.dev/development/platform-integration/windows/building#integrating-with-windows
 [platform channels]: https://docs.flutter.dev/development/platform-integration/platform-channels
-[interop example]: https://github.com/flutter/flutter/tree/master/examples/platform_channel
+[interop example]: https://github.com/flutter/flutter/tree/main/examples/platform_channel
 [Impeller]: https://docs.flutter.dev/perf/impeller