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move feature gate checks inside IsCriticalPod

Currently `IsCriticalPod()` calls throughout the code are protected by `utilfeature.DefaultFeatureGate.Enabled(features.ExperimentalCriticalPodAnnotation)`.

However, with Pod Priority, this gate could be disabled which skips the priority check inside IsCriticalPod().

This PR moves the feature gate checking inside `IsCriticalPod()` and handles both situations properly.

@aveshagarwal @ravisantoshgudimetla @derekwaynecarr 
/sig node
/sig scheduling
/king bug