What does this MR do?
The refactored boards, which will roll out in this milestone, doesn't support multi select drag&drop.
Users will get confused when they can multi select issue cards but cannot use drag&drop so we decided to temporarily disable multi select by putting it behind a feature flag.
For more context, please see #289797
The new version of boards uses Draggable to drag and drop an issue.
Once Draggable supports multi drag (https://github.com/SortableJS/Vue.Draggable/pull/744), it should be added to GraphQL boards.
Does this MR meet the acceptance criteria?
I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
I have added/updated documentation, or it's not needed. (Is documentation required?)
I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
I have self-reviewed this MR per code review guidelines.
This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
I have followed the style guides.