Move pipelines from widget.json to cached_widget.json

What does this MR do?

Since we already have pipelines expiration mechanism and it's a low effort to extend it with one more route, what about trying to move some of the pipelines data from widget.json into cached_widget.json?

In this case, we won't calculate the data every time widget.json is fetched, it will be cached using Etag caching and invalidated by https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/services/ci/expire_pipeline_cache_service.rb

Related issue: gitlab-com/gl-infra/scalability#481 (closed)