[k8sattributes] Mature `k8sattr.labelsAnnotationsSingular.allow` feature gate

[ChrsMark]

Component(s)

processor/k8sattributes

Describe the issue you're reporting

With #39775 the k8sattr.labelsAnnotationsSingular.allow feature gate was introduced to fix #39774.

This issue keeps track the work needed to move this feature gate through maturity levels and eventually remove it.

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[github-actions]

Pinging code owners:

• processor/k8sattributes: @dmitryax @fatsheep9146 @TylerHelmuth @ChrsMark

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