[97411]: Display preferred name in authenticated menu when available

[dcloud]

Did you change site-wide styles, platform utilities or other infrastructure?

• No
• Yes, and I used the proxy-rewrite steps to test the injected header scenario

Summary

• Now that preferred name is being parsed from MPI, use preferred name in site-wide nav when available

Related issue(s)

• PrefName: Display prefName in auth menu when available va.gov-team#97411
• https://jira.devops.va.gov/browse/VI-771,
  per [VI-771] Adding preferred name to user serializer, sourced from MPI vets-api#19488

Testing done

• Describe what the old behavior was prior to the change
• Describe the steps required to verify your changes are working as expected
• Describe the tests completed and the results
• _Exclusively stating 'Specs and automated tests passing' is NOT acceptable as appropriate testing
• Optionally, provide a link to your test plan and test execution records

Screenshots

Note: This field is mandatory for UI changes (non-component work should NOT have screenshots).

	Before	After
Mobile
Desktop

What areas of the site does it impact?

(Describe what parts of the site are impacted if code touched other areas)

Acceptance criteria

Quality Assurance & Testing

• I fixed|updated|added unit tests and integration tests for each feature (if applicable).
• No sensitive information (i.e. PII/credentials/internal URLs/etc.) is captured in logging, hardcoded, or specs
• Linting warnings have been addressed
• Documentation has been updated (link to documentation *if necessary)
• Screenshot of the developed feature is added
• Accessibility testing has been performed

Error Handling

• Browser console contains no warnings or errors.
• Events are being sent to the appropriate logging solution
• Feature/bug has a monitor built into Datadog or Grafana (if applicable)

Authentication

• Did you login to a local build and verify all authenticated routes work as expected with a test user

Requested Feedback

(OPTIONAL) What should the reviewers know in addition to the above. Is there anything specific you wish the reviewer to assist with. Do you have any concerns with this PR, why?