MR pipelines GraphQL refactor and subscription

Proposal

• Finish the work started with the mr_pipelines_graphql feature flag.
• Add a subscription
• Cleanup old code

Technical notes

• App injection happens in app/assets/javascripts/merge_request_tabs.js based on FF state.
• Need to add a subscription and replace polling in app/assets/javascripts/ci/merge_requests/components/pipelines_table_wrapper.vue
• Need to ensure app/assets/javascripts/ci/merge_requests/components/pipelines_table_wrapper.vue is completely backwards compatible with commit/pipelines/legacy_pipelines_table_wrapper.vue
• app/assets/javascripts/ci/merge_requests/components/pipelines_table_wrapper.vue still formats data to REST for the pipelines table but after the work in #223264 the table and downstream components are setup to support GraphQL format.