feat(dropdowns): migrate to Floating UI

This drops Popper.js in favor of its successor, Floating UI.

BREAKING CHANGE: While Floating UI is similar to Popper.js, its API
changed slightly. It might be worth reading the migration guide over at
https://floating-ui.com/docs/migration.

`GlDisclosureDropdown` and `GlListbox` previously exposed a way to
customize Popper.js' config through the `popperOptions` prop. This has
been deprecated in favor of a `dropdownOffset` prop which accepts the
same values as Floating UI's `offset` middleware.
https://floating-ui.com/docs/offset