feat: remove default 'ubuntu' user on Ubuntu 23.04+

[edwardzjl]

Starting from Ubuntu 23.04, the system includes a default 'ubuntu' user (UID 1000) to serve as a non-root OCI user. This cause errors when building the base image: useradd: UID 1000 is not unique.

Since we don't need the default 'ubuntu' user, this commit removes it.

Original PR: kubeflow/kubeflow#7728

[google-oss-prow]

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[andyatmiami]

/area v1

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