Add unmaintained notice to contrib basic-auth

[annajung]

Signed-off-by: Anna Jung (VMware) [email protected]

Which issue is resolved by this Pull Request:
Part of #2311

Description of your changes:
Add README to /contrib/basic-auth to mark the directory as unmaintained and out of date

Note:
The OWNERS file is not available for this component, therefore, am not sure who the original owners are. Tagging @yanniszark (last commit) to ensure the component OWNER(s) are aware of the out-of-date label put on the component.

To remove the unmaintained label, check out the missing requirements #2311 and leave a comment on the issue to indicate that you'll take action to provide the missing requirements and are willing to keep the component up to date.

cc @kimwnasptd @kubeflow/wg-manifests-leads

[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: annajung
Once this PR has been reviewed and has the lgtm label, please assign kimwnasptd for approval by writing /assign @kimwnasptd in a comment. For more information see the Kubernetes Code Review Process.

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[annajung]

closing in favor of #2313