chore: Add securitycontext for PSS PoC (rootless Kubeflow)

[biswassri]

Pull Request Template for Kubeflow manifests Issues

✏️ A brief description of the changes

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Adding PSS enforceable for sso and oauth2-proxy and enjoy a rootless Kubeflow #2528

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• Make sure you have tested with kustomize. See Installation Prerequisites
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[juliusvonkohout]

This is an upstream file. Please do not modify it, but instead our DEX at https://github.com/kubeflow/manifests/tree/master/common/dex

[biswassri]

Sure! I'll update this.

[google-oss-prow]

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[juliusvonkohout]

We also need to remove the two patches from /contrib/security then if we apply them always.