Change blob_overflow_menu feature flag type

• Change blob_overflow_menu feature flag to gitlab_com_derisk type
• Create roll-out issue
• Adjust documentation about blob controls
  • https://docs.gitlab.com/ee/user/project/repository/web_editor.html
  • https://docs.gitlab.com/ee/user/project/file_lock.html
  • replace, and delete a file
  • view file history
• Adjust selectors in feature tests (after changing buttons to dropdown items)