feat(GlTokenSelector): Add `disabled` prop

What does this MR do?

Adds a disabled prop to GlTokenSelector component. Needed for gitlab#22754 (comment 503838015)

Enabled

Disabled

Note about visual regression tests

It looks like visual regression tests were disabled in a614c100 because they were flaky. I took a look at the screenshots and it appears they were flaky because the input was focused and therefore the pulsing text cursor would sometimes be in the screenshot and sometimes not.

Unfortunately I don't think there is anyway to avoid the pulsing text cursor so I removed this line. The dropdown won't be in the snapshot but I figured a less comprehensive test is better than no test at all.

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: GlTokenSelector not used
  • Status Page: GlTokenSelector not used
• 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.