Wrong fallback placement for dropdown

Created by: rohit2sharma95

When positioned with Popper, fallback placement for dropdown seems wrong.

Current behavior (Flipped with top placement)	Expected behavior (Flipped with top-end placement)

Because the changes made in #32405 override the Popper's default value for fallbackPlacement that is [oppositePlacement]. By default Popper places the element to the opposite side (like bottom-end => top-end and bottom => top).

Now Popper can place the element to the specified positions only: https://github.com/twbs/bootstrap/blob/122b2bdd485373c05eeab50c9f809db31117b898/js/src/dropdown.js#L311-L316

But these changes are good for Tooltip/Popover (Because they are not placed to the edges of the reference element)