chore(docs): bootstrap and ng-bootstrap removal in migration guide

[leagrdv]

📄 Description

Updated the migration guide:

• Added information regarding the removal of bootstrap and ng-bootstrap in one place
• Cleaned up scattered informations regarding bootstrap removal in the rest of the page
• Removed all informations of removed elements that were never previously documented

📝 Checklist

• ✅ My code follows the style guidelines of this project
• 🛠️ I have performed a self-review of my own code
• 📄 I have made corresponding changes to the documentation
• ⚠️ My changes generate no new warnings or errors
• 🧪 I have added tests that prove my fix is effective or that my feature works
• ✔️ New and existing unit tests pass locally with my changes

[changeset-bot]

🦋 Changeset detected

Latest commit: fc0e04f

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@swisspost/design-system-documentation	Minor

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[swisspost-bot]

Related Previews

• https://preview-6208--swisspost-design-system-next.netlify.app

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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