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.

Update documentation for e2e testing

See merge request !4037



Merged-by: Paul Gascou-Vaillancourt <pgascouvaillancourt@gitlab.com>
Approved-by: David Pisek <dpisek@gitlab.com>
Approved-by: Paul Gascou-Vaillancourt <pgascouvaillancourt@gitlab.com>
Reviewed-by: Paul Gascou-Vaillancourt <pgascouvaillancourt@gitlab.com>
Co-authored-by: Grace <gcw.chung@gmail.com>

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

Closes #2578

See merge request gitlab-org/gitlab-ui!4066



Merged-by: Olena Horal-Koretska <ohoralkoretska@gitlab.com>
Approved-by: Paul Gascou-Vaillancourt <pgascouvaillancourt@gitlab.com>
Approved-by: Olena Horal-Koretska <ohoralkoretska@gitlab.com>
Co-authored-by: Miguel Rincon <mrincon@gitlab.com>

# [78.12.0](gitlab-org/gitlab-ui@v78.11.0...v78.12.0) (2024-04-10)

### Features

* **GlBaseDropdown:** Migrates "placement" values to match floating-ui placements ([a9f37b6b](gitlab-org/gitlab-ui@a9f37b6b))