This PR makes sure the manifests under `content/en/examples`
are valid in v1.28. The primary fixes are:
- Updated the go.mod/go.sum file for testing against v1.28.0 release.
- Revise test case code to ensure newly added manifests are tested;
- Adapt Pod validation options to upstream validation code change;
- Move a ValidatingWebhookConfiguration YAML back to inline because
the manifest cannot validate against the validator. The CA bundle
referenced is not a valid string (base64 encoded). That means the
YAML cannot be used/tested as is by users.
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