Introduce board multi select feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

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?

Conformity

• 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?)
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• 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.