Wrong fallback placement for dropdown

[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:

bootstrap/js/src/dropdown.js

Lines 311 to 316 in 122b2bd

	{
	name: 'flip',
	options: {
	fallbackPlacements: ['top', 'right', 'bottom', 'left']
	}
	}]

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