ProgressBar: Use gray 300 for track color

[tyxla]

What?

This PR updates the ProgressBar indicator color to use the gray 300 instead of gray 100 for the progress bar track.

Why?

Improves the color combinations when used in practice in #53032.

How?

We're just adjusting the indicator color to use the gray 300 instead of gray 100 for the progress bar track.

Testing Instructions

• Run npm run storybook:dev
• Navigate to http://localhost:50240/?path=/docs/components-experimental-progressbar--default
• Verify the progress bar works and looks well.
• Play with the Theme controls (the pencil icon) and verify the combinations work well.
• Test it with Edit Site: Use progress bar for loading screen #53032.

Testing Instructions for Keyboard

None

Screenshots or screencast

Before:

After: