Fix FollowRecommendationsScheduler failing because of unpopulated views (#16189)

Since #16173, `account_summaries` and `follow_recommendations` are not
populated at creation time, which causes concurrent refresh to fail.

As we currently only use those materialized views right after explicitly
refreshing them, this commit changes refreshes to not be performed
concurrently. This will fix the issue and ensure the refresh completes
faster while using less resources.