chore: change branch master to main in github configurations (googleapis#1465)

Author: Neenu1995

.github/blunderbuss.yml

@@ -1,5 +1,5 @@
 # Configuration for the Blunderbuss GitHub app. For more info see
-# https://github.com/googleapis/repo-automation-bots/tree/master/packages/blunderbuss
+# https://github.com/googleapis/repo-automation-bots/tree/main/packages/blunderbuss
 assign_prs_by:
 - labels:
   - samples

.github/workflows/ci.yaml

@@ -1,7 +1,7 @@
 on:
   push:
     branches:
-    - master
+    - main
   pull_request:
 name: ci
 jobs: