Fix placeholder shortcode color and background params when 'img' markup

[julien-deramond]

Description

In the placeholder shortcode, when the markup parameter is set to img:

• Text color is fixed: fill='%23868e96'
• Background color is fixed: fill='%23dee2e6'
  So the shortcode is not really dynamic in this case.

OK... it is only used for now in "Object fit" page with fixed colors. But still, I'm suggesting this change 😇

Motivation & Context

Make placeholder shortcode really react to dynamic text and background colors.

Type of changes

• Refactoring (non-breaking change)

Checklist

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

Live previews

• https://deploy-preview-38263--twbs-bootstrap.netlify.app/docs/5.3/utilities/object-fit/ (for non-regression testing)