Update Portal.stories.tsx to no longer use styled-components

[Copilot]

This PR migrates packages/react/src/Portal/Portal.stories.tsx away from styled-components as part of our effort to remove references and usage of the library in Storybook stories.

Changes Made

• Removed Box component dependency: Replaced <Box bg="red.2" p={3}> and <Box bg="green.2" p={3}> with standard <div> elements
• Created CSS module: Added Portal.stories.module.css with properly scoped styles using Primer design tokens
• Updated color variables: Used --bgColor-danger-muted and --bgColor-success-muted instead of the deprecated --base-color-red-2 and --base-color-green-2 tokens to comply with Primer color guidelines
• Maintained visual consistency: The story continues to display the same red and green container layout as before

The Portal story now uses CSS modules for styling instead of styled-components, while preserving the exact same visual appearance and functionality.

Screenshot

The story shows the proper red outer container and green inner container, with the portal content being rendered at the BaseStyles root as expected.

Fixes #6579.

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