fix(ng-dev/release): wait for next branch update action PR to be merged

Instead of just creating the `switch-next-to-major` PR, we should also
prompt for merge and wait for it to be merged. This ensures the
caretaker does not get into bad state and completes the release action
as expected.

ng-dev/release/publish/actions/configure-next-as-major.ts

@@ -45,6 +45,8 @@ export class ConfigureNextAsMajorAction extends ReleaseAction {
     );
 
     Log.info(green('  ✓   Next branch update pull request has been created.'));
+
+    await this.promptAndWaitForPullRequestMerged(pullRequest);
   }
 
   static override async isActive(active: ActiveReleaseTrains) {

ng-dev/release/publish/test/configure-next-as-major.spec.ts

@@ -69,7 +69,9 @@ describe('configure next as major action', () => {
       .expectBranchRequest('master', 'MASTER_COMMIT_SHA')
       .expectCommitStatusCheck('MASTER_COMMIT_SHA', 'success')
       .expectFindForkRequest(fork)
-      .expectPullRequestToBeCreated('master', fork, expectedForkBranch, 200);
+      .expectPullRequestToBeCreated('master', fork, expectedForkBranch, 200)
+      .expectPullRequestMergeCheck(200, false)
+      .expectPullRequestMerge(200);
 
     // In the fork, we make the staging branch appear as non-existent,
     // so that the PR can be created properly without collisions.