feat(GlBaseDropdown): Migrates "placement" values to match floating-ui placements

This change adds 'bottom-start', 'bottom' and 'bottom-end' as
allowed placements for the GlListbox and GlDisclosureDropdown.

The default value is "bottom-start".

Usage:

```
<gl-disclosure-dropdown placement="bottom-end">
</gl-disclosure-dropdown>
```

If also deprecates "left", "center", "right" in favor of those
placements respectively.