Fix insufficient contrast on borders

[GhassenRjab]

Description

Use $gray-600 for border color in light mode.
This has a contrast of 4.69 to white, which should be good

Motivation & Context

This solves the insufficient contrast on borders when on light mode. Issue opened here: #38480

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-40949--twbs-bootstrap.netlify.app/

Related issues

No related issues.

[julien-deramond]

Thanks for your PR @GhassenRjab, but the suggested value is too dark. A lighter one was already refused in #38489.
As said here #38489 (comment):

Let's keep the PRs at bay for now and discuss any potential changes in the issue. I could see us darkening things a little, but this is too much.