fix(GlDropdown): Fix GlDropdown not auto flipping horizontally when it would overflow

What does this MR do?

This MR fixes an issue where opening a GlDropdown will auto-flip the dropdown menu vertically (top to bottom and vice versa) when it would spill out of the parent element, but not horizontally (left to right and vice versa). The bug was fixed in Bootstrap 5, but the BootstrapVue library is still on Bootstrap 4, so this MR adds a fix to make it flip horizontally to match the Bootstrap 5 behavior.

Before	After

Note that Bootstrap 4 will shift the dropdown menu to the left rather than flipping it to avoid the right edge of the parent, but Bootstrap 5 will flip it instead. Also note that in Storybook it's shifted enough to avoid spilling out, but in the GitLab project on the vulnerability report, it doesn't shift it over enough and it will spill out, which is fixed by adding in the horizontal auto-flip:

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.

Closes #1965 (closed)