Change default branch name from master to main

Author: r7kamura

.github/workflows/github-label-sync.yml

@@ -3,7 +3,7 @@ name: github-label-sync
 on:
   push:
     branches:
-      - master
+      - main
     paths:
       - .github/workflows/github-label-sync.yml
   workflow_dispatch:

.github/workflows/release.yml

@@ -3,7 +3,7 @@ name: release
 on:
   push:
     branches:
-      - master
+      - main
 
 jobs:
   release:

.github/workflows/test.yml

@@ -4,7 +4,7 @@ on:
   pull_request:
   push:
     branches:
-      - master
+      - main
 
 jobs:
   rspec: