Add A11y test for GlNav component

What does this MR do?

Implements the following accessibility test for the GlNav component:

• default with dropdown item

A11y violations to be addressed

Violation	Fix
Text elements must have sufficient color contrast against the background	!4050 (closed) (7602f390)
Certain ARIA roles must be contained by particular parent elements
<ul> and <ol> must only directly contain <li>, <script> or <template> elements
Links must have discernible text It seems that nav_item_dropdown - text prop needs to be required to meet requirements

GlNav storybook violations

Additional changes

• Minor update to GlNav storybook to "fix" a11y violation - links must have discernible text
• Add nav link default and hover text colors to address color contrast violation

Screenshots or screen recordings

Color contrast

Before	After

Integration merge requests

• GitLab: mr_url
• CustomersDot: mr_url
• Status Page: mr_url

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

Toggle the acceptance checklist

Conformity

• Code review guidelines.
• GitLab UI's contributing guidelines.
• 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 (see integration merge requests above).
• 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 #2496 (closed)