feat(Breadcrumb): Update design

What does this MR do?

• Update the component to align with the recent update to the design specs (gitlab-org/gitlab-services/design.gitlab.com#1533 (closed)).
• Remove the <slot> that allows a custom separator between breadcrumb items. As of yet, this hasn't been a design requirement and we haven't promoted custom separators. It's not part of the latest design spec either. This change lightens up the CSS and allows us to use a pseudo class for the separator instead of including an additional element in the <li> that would have to be hidden from assistive technology. In other words, it's a reduction in both the CSS and resulting markup.
• Fixes the list semantics by placing the ellipse button (in the collapsed items example) in a <li> rather than having a button inserted in between list items.
• Update the default avatar image from a picture of Sid to the GitLab logomark. Not only does this align with the design, but it also is more accurate in representing the root level.

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 #2141 (closed)