style(pds-progress): update progress bar background color styles

[pixelflips]

Description

This PR enables CSS gradient support for the pds-progress component's fill color by changing the background-color property to background in the progress bar styling.

Previously, users could not use gradients (e.g., linear-gradient(), radial-gradient()) with the fillColor prop or --color-progress-fill CSS variable because background-color only accepts solid color values. This change maintains backward compatibility with solid colors while unlocking support for gradients and other advanced background properties.

Fixes: https://kajabi.atlassian.net/browse/DSS-1560

Changes Made

• Updated pds-progress.scss to use background instead of background-color for WebKit and Mozilla progress bar pseudo-elements
• Added documentation example demonstrating gradient usage with Pine color tokens

Screenshot

Type of change

• Style fix (non-breaking change which fixes an issue)
• This change requires a documentation update

How Has This Been Tested?

• tested manually - Verified gradient rendering in progress component
• Tested solid color compatibility to ensure no regression
• Validated documentation example displays correctly

Test Configuration:

• Pine versions:
• OS:
• Browsers:
• Screen readers:
• Misc:

Checklist:

If not applicable, leave options unchecked.

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes
• Design has QA'ed and approved this PR

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[QuintonJason]

Good stuff