Rename MWBS service to Merge When Pipeline Succeeds ## What does this MR do? This MR renames Merge When Build Succeeds feature to Merge When Pipeline Succeeds, because we recently changed the behavior, see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6675. ## Does this MR meet the acceptance criteria? - [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) entry added - [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md) - [x] All builds are passing ## What are the relevant issue numbers? Closes #23354 See merge request !7135