Skeleton -- update gradient to use new skeleton color tokens

We added new design tokens to support skeleton gradients via https://github.com/patternfly/design-tokens/issues/66.

We'll need to update the skeleton component's gradient to use the new tokens. 

Designs: 

<img width="788" alt="Image" src="https://github.com/user-attachments/assets/a7279177-0721-484d-9f75-19714ebd4b61" />

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**Jira Issue:** [PF-1762](https://issues.redhat.com/browse/PF-1762)