feat(v5): Drop InputGroupPrepend and InputGroupAppend

[kyletsang]

https://v5.getbootstrap.com/docs/5.0/forms/input-group/

Note:
I did run into an issue with the dropdowns that might need to be solved in another PR. The new markup for dropdowns has the dropdown toggle and menu directly under the InputGroup.

In v4 this was solved with <DropdownButton as={InputGroup.Prepend}>, but there's no parent element within InputGroup this time.

I don't think there's a way to solve this with the existing components... anybody have any suggestions?

Changes:

• Drop InputGroupPrepend and InputGroupAppend
• Replace input in InputGroupCheckbox with FormCheckInput
• Fix tests and examples

[jquense]

I did run into an issue with the dropdowns that might need to be solved in another PR. The new markup for dropdowns has the dropdown toggle and menu directly under the InputGroup.

react-bootstrap/react-overlays#843 may unlock some API improvements there, since we are trying to remove the need for Dropdown to render a wrapping element

[kyletsang]

Thanks @jquense. I'll merge this one first.