Fix: Ensure menu is always in front of everything

[GuySartorelli]

Fixes https://github.com/JedWatson/react-select/issues/5329

It's not uncommon for elements to have a z-index of >1, which would put them in front of the dropdown menu. There's no valid use case that I can think of where you'd actually want something to be in front of the dropdown menu, so the menu should have a really high z-index value to ensure it's on top of everything all the time.

In the unlikely event someone wants a specific element to be in front of the menu, they can either give that element a really high z-index, or change the z-index of menu for their specific use case - but we shouldn't have to customise the z-index to get the menu to display on top. The menu being on top should be the default behaviour.

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[GuySartorelli]

Let me know if you want any tests, docs, etc - I didn't see a contribution guide so I just went with the least invasive change.