Bump @headlessui/react to v2

[RobinMalfait]

This PR bumps @headlessui/react to the latest v2 version.

There are 3 relevant changes at play here:

1. Bump the actual @headlessui/react version.
2. Remove the as={Fragment} prop from the <Transition> and <Transition.Child> components. This is now the default behavior.
3. Replace the deprecated dot notation with concrete imported components.

These changes are optional because they work in a backwards compatible way. But since the as={Fragment} prop is now the default behavior, it's cleaner to remove it.

The dot notation will also still work, but since it's deprecated, your editor will show a warning, so let's clean it up as well.

Apart from these steps nothing else is changed nor required.

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[taylorotwell]

Thanks!