Update azure-pipelines.yml to use ubuntu-20.04 (#16485)

* Update azure-pipelines.yml

* Update ShouldSendPRToMain.yml

* Update BranchProtectionForPrivateRepo.yml

.azure-pipelines/BranchProtectionForPrivateRepo.yml

@@ -1,7 +1,5 @@
 jobs:
   - job: "BranchProtectionForPrivateRepo"
-    pool:
-      vmImage: 'Ubuntu 16.04'
     condition: and(variables['PRIVATE'], eq(variables['System.PullRequest.TargetBranch'], 'main'))
     steps:
     - script: '>&2 echo "the branch is protected"'

.azure-pipelines/ShouldSendPRToMain.yml

@@ -1,7 +1,5 @@
 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"'

azure-pipelines.yml

@@ -1,5 +1,8 @@
 name: "Azure OpenAPI"
 
+pool:
+  vmImage: 'Ubuntu-20.04'
+
 trigger:
   branches:
     include: