Docs: fix desc around deprecated Sass mixins for alerts and list groups

[julien-deramond]

Description

This PR fixes the Alerts and List group documentation pages based on the deprecation of their respective alert-variant() and list-group-item-variant() mixins:

• in the Sass loops section, don't mention anymore the mixin but the override of CSS variables
• in the Sass mixins section, drop entirely the sentence that isn't true anymore

This PR also fixes some minor issues in the migration guide:

• Drop the dot prefixing the name of the mixins
• Use the right name for the deprecated list group mixin

Type of changes

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

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

Live previews

• https://deploy-preview-39537--twbs-bootstrap.netlify.app/docs/5.3/components/alerts/#sass-mixins
• https://deploy-preview-39537--twbs-bootstrap.netlify.app/docs/5.3/components/alerts/#sass-loops
• https://deploy-preview-39537--twbs-bootstrap.netlify.app/docs/5.3/components/list-group/#sass-mixins
• https://deploy-preview-39537--twbs-bootstrap.netlify.app/docs/5.3/migration/