Docs: rollback some modification about `.text-muted` deprecation messages

Description

https://github.com/twbs/bootstrap/commit/9042efd0f86dd571386f62bacc8738ac975517fc introduced some issues with the deprecation messages about .text-muted. This PR fixes these messages.

Type of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the contributing guidelines
• (N/A) 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
• (N/A) I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-38152--twbs-bootstrap.netlify.app/docs/5.3/migration/#utilities-1
• https://deploy-preview-38152--twbs-bootstrap.netlify.app/docs/5.3/utilities/colors/#colors