Remove p-link--external

[sowasred2012]

Done

• Removed .p-external--link and associated examples
• Added note about removal to upgrade guide

Fixes #3375

QA

• Open demo
• See that external links aren't mentioned
• Click around https://vanilla-framework-4069.demos.haus and similar pages, see that no links have the icon
• Check the codebase for mentions of p-link--external, there should be none.
  • the exception is for the icon itself, p-icon--external-link still exists
• Review updated documentation:
  • Upgrade guide

Check if PR is ready for release

If this PR contains Vanilla SCSS code changes, it should contain the following changes to make sure it's ready for the release:

• PR should have one of the following labels to automatically categorise it in release notes:
  • Feature 🎁, Breaking Change 💣, Bug 🐛, Documentation 📝, Maintenance 🔨.
• Vanilla version in package.json should be updated relative to the most recent release, following semver convention:
  • if CSS class names are not changed it can be bugfix relesase (x.x.X)
  • if CSS class names are changed/added/removed it should be minor version (x.X.0)
  • see the wiki for more details
• Any changes to component class names (new patterns, variants, removed or added features) should be listed on the what's new page.
• Documentation side navigation should be updated with the relevant labels.

[webteam-app]

Demo starting at https://vanilla-framework-4069.demos.haus

[bartaz]

LGTM