set default branch as main for pipeline (#15465)

* set default branch as main

* block pr to public spec repo

.azure-pipelines/BranchProtectionForPrivateRepo.yml

@@ -2,7 +2,7 @@ jobs:
   - job: "BranchProtectionForPrivateRepo"
     pool:
       vmImage: 'Ubuntu 16.04'
-    condition: and(variables['PRIVATE'], eq(variables['System.PullRequest.TargetBranch'], 'master'))
+    condition: and(variables['PRIVATE'], eq(variables['System.PullRequest.TargetBranch'], 'main'))
     steps:
     - script: '>&2 echo "the branch is protected"'
       failOnStderr: true

.azure-pipelines/ShouldSendPRToMain.yml

@@ -0,0 +1,8 @@
+jobs:
+  - job: "ShouldSendPRToMain"
+    pool:
+      vmImage: 'Ubuntu 16.04'
+    condition: eq(variables['System.PullRequest.TargetBranch'], 'master')
+    steps:
+    - script: '>&2 echo "##vso[task.logissue type=error]Please edit the pull request to targeting new default branch main"'
+      failOnStderr: true

azure-pipelines.yml

@@ -18,4 +18,5 @@ variables:
   PR_ONLY: 'true'
 
 jobs:
-- template: .azure-pipelines/BranchProtectionForPrivateRepo.yml
+- template: .azure-pipelines/BranchProtectionForPrivateRepo.yml
+- template: .azure-pipelines/ShouldSendPRToMain.yml