Fix Popper preventOverflow boundary config

[kyletsang]

Currently, the boundary config is being assigned to the wrong var (rootBoundary) in the popper config. It should be assigned to the boundary var in popper's config.

Ref: https://popper.js.org/docs/v2/utils/detect-overflow/#boundary

[XhmikosR]

@kyletsang do we have any issue about this? I don't suppose this issue could be related to any of the other Popper issues we have in v5?

[kyletsang]

@XhmikosR, currently no issue reported. I was reviewing the code to see if there's any new modifiers I needed to port over to react-bootstrap when I stumbled on it.

This could potentially cause issues due to incorrect usage of rootBoundary. Unfortunately, I haven't tested in-depth though.

[XhmikosR]

@kyletsang do you think you could add some tests about this?

[kyletsang]

Ugh, I think I'm going to need some assistance on this. I can't seem to come up with a demo that shows any difference when setting an incorrect rootBoundary vs a correct one.

[XhmikosR]

Unfortunately, I have no idea how to test for this. We should have a test to make sure we don't hit this again, but if we can't have a test, I guess we could make an exception here.

[XhmikosR]

@rohit2sharma95 LGTY?