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

.github/workflows/easy-cp.yml

@@ -53,7 +53,7 @@ jobs:
       # TODO(xilaizhang): remove this step once the template is available on release branches.
       - name: Get CP Template
         run: |
-          curl -o PULL_REQUEST_CP_TEMPLATE.md https://raw.githubusercontent.com/flutter/flutter/master/.github/PR_TEMPLATE/PULL_REQUEST_CP_TEMPLATE.md
+          curl -o PULL_REQUEST_CP_TEMPLATE.md https://raw.githubusercontent.com/flutter/flutter/main/.github/PR_TEMPLATE/PULL_REQUEST_CP_TEMPLATE.md
       - name: Create PR on CP success
         if: ${{ steps.attempt-cp.conclusion == 'success' }}
         working-directory: ./flutter