graduate ServiceIPStaticSubrange to beta

[aojea]

/kind feature

graduate ServiceIPStaticSubrange feature to beta (disabled by default)

It seems that the feature had some external users testing and found an important bug

#109928

since then, no more issues were reported matching the criteria

https://github.com/kubernetes/enhancements/tree/master/keps/sig-network/3070-reserved-service-ip-range#beta

Ref: kubernetes/enhancements#3070

[k8s-ci-robot]

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[aojea]

/hold
/assign @liggitt @thockin

[aojea]

/sig network
holding meanwhile is not discussed and added to the enhancements track

[thockin]

/lgtm
added to spreadsheet

[dcbw]

/approve

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