Reland: Update link branches to main (#146882)

Reland https://github.com/flutter/flutter/pull/146558, reverted in https://github.com/flutter/flutter/pull/146880 due to an outdated test result

## Original description

- 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

.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