Enhance content negotiation for zpages

[richabanker]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Replace explicit switch statements with generic media type handling that automatically supports all registered serializers (JSON, YAML). This prevents erroneous 500 http code returned before for Accept: application/yaml;v=v1alpha1;g=config.k8s.io;as=Statusz
Also, rejects protobuf since zpages types lack protobuf definitions.

Which issue(s) this PR is related to:

kubernetes/enhancements#4827
kubernetes/enhancements#4828

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: richabanker

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Needs approval from an approver in each of these files:

• staging/src/k8s.io/apiserver/pkg/server/flagz/OWNERS [richabanker]
• staging/src/k8s.io/apiserver/pkg/server/statusz/OWNERS [richabanker]

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