feat(skeletons): added on-secondary color

[agliga]

Fixes #2305

• This PR contains CSS changes
• This PR does not contain CSS changes

Description

• Added skeleton--on-secondary modifier
• Updated tokens for both base skeleton colors but also for on-secondary
• As per design --elevated is deprecated. The colors in on-secondary should be used in --elevated

Screenshots

Checklist

• I verify the build is in a non-broken state
• I verify all changes are within scope of the linked issue
• I regenerated all CSS files under dist folder
• I tested the UI in all supported browsers
• I did a visual regression check of the components impacted by doing a Percy build and approved the build
• I tested the UI in dark mode and RTL mode
• I added/updated/removed Storybook coverage as appropriate

[changeset-bot]

🦋 Changeset detected

Latest commit: 18a46ff

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@ebay/skin	Minor

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[agliga]

Question, should we call this on-secondary or change it to simply secondary?

[ArtBlue]

LGTM!