Refactor panel height calc with CSS only

What does this MR do and why?

Refactor panel height for issue boards and issue bulk sidebar to use flex

Screenshots or screen recordings

Issue boards

Using only flex will do the trick

Before	After

1. Go to https://gdk.test:3000/flightjs/Flight/-/boards/3

Issue bulk sidebar

Moving to flex will do the trick

Before	After

1. Go to https://gdk.test:3000/flightjs/Flight/-/issues
2. Click on Bulk edit

MR acceptance checklist

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Related to #548836 (closed)