Restore missing scrollbar on boards

What does this MR do?

Fix bug introduced in !53690 (merged)

When on boards page, sometimes the horizontal scrollbar is hidden below the bottom of the page and not clickable. This happens if the filter bar has enough items that it has a scrollbar, or if there is a flash message displayed.

We re-add overflow-auto to the body. This does mean we have double scrollbars on the right side again, will address this in #322086 (closed).

before	after

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
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