fix: panic in util.GVRFromType for core objects

[L3n41c]

What this PR does / why we need it:

Fixes a panic in the util.GVRFromType function when called for a core object.

For a pod object, for example, the apiVersion would be v1,

expectedTypeSlice := strings.Split(apiVersion, "/")

would return a slice of a single element because apiVersion has no /.
And

v := expectedTypeSlice[1]

would panic because of an array out-of-bound access.

How does this change affect the cardinality of KSM: (increases, decreases or does not change cardinality)

Does not change.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

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[CatherineF-dev]

Core resource metrics are exposed using these functions https://github.com/kubernetes/kube-state-metrics/tree/main/internal/store

[mrueg]

Core resource metrics are exposed using these functions https://github.com/kubernetes/kube-state-metrics/tree/main/internal/store

Yes, that's correct. We keep core resource metrics internal.
I still agree that the function shouldn't panic when used externally (or we should move the whole code into internal)

/lgtm

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: L3n41c, mrueg

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