build: add mainBranchName to github configuration (#43061)

Update the ng-dev config for Github to include the main branch name.

PR Close #43061

.ng-dev/github.ts

@@ -4,8 +4,8 @@ import {GithubConfig} from '@angular/dev-infra-private/ng-dev/utils/config';
  * Github configuration for the `ng-dev` command. This repository is used as
  * remote for the merge script and other utilities like `ng-dev pr rebase`.
  */
-
 export const github: GithubConfig = {
   owner: 'angular',
-  name: 'angular'
+  name: 'angular',
+  mainBranchName: 'master',
 };