Add joaquimrocha as a SIG UI lead

[joaquimrocha]

• One-line PR description: Adding myself (@joaquimrocha) as a SIG UI lead.

• Issue link: SIG UI: Leadership Change community#8652

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: joaquimrocha
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[shu-mutou]

LGTM