build: use mjs files for ng-dev configuration

In preperation for ng-dev requiring mjs files for configuration, switch over our usage

.ng-dev/github.mjs

@@ -0,0 +1,12 @@
+/**
+ * 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`.
+ *
+ * @type { import("@angular/ng-dev").GithubConfig }
+ */
+export const github = {
+  owner: 'angular',
+  name: 'angular',
+  mainBranchName: 'main',
+  useNgDevAuthService: true,
+};