Update more links and mentions to the main branch (#1294)

.github/PULL_REQUEST_TEMPLATE.md

@@ -17,4 +17,4 @@ If you need help, consider asking for advice on the #hackers-devrel channel on [
 [Flutter Style Guide]: https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
 [CLA]: https://cla.developers.google.com/
 [Discord]: https://github.com/flutter/flutter/wiki/Chat
-[Contributors Guide]: https://github.com/flutter/samples/blob/master/CONTRIBUTING.md
+[Contributors Guide]: https://github.com/flutter/samples/main/master/CONTRIBUTING.md

.github/workflows/beta.yml

@@ -16,7 +16,7 @@ defaults:
 
 jobs:
   # Run the stable test script on the beta channel. Since this branch will soon
-  # be merged into master as our stable-targeting code, this is the key thing we
+  # be merged into main as our stable-targeting code, this is the key thing we
   # need to test.
   stable-tests-on-beta:
     runs-on: ${{ matrix.os }}

.github/workflows/main.yml

@@ -1,4 +1,4 @@
-name: Master Branch CI
+name: Main Branch CI
 
 # Declare default permissions as read only.
 permissions: read-all