Update to Angular@20

[oliverschuerch]

As a maintainer, I want to update the components-angular package to Angular 20, so that our design system stays up-to-date and compatible with the latest Angular version.

• The demo app package must be removed first (chore: remove demo app from v9 #6099).
• The components-angular package can/should be updated to Angular 20 by cherry-picking commit b5e7e61.
• The intranet-header package must be updated at the same time, to ensure compatibility with Angular 20.
• The monorepo builds successfully locally and on CI after the update.
• A proper changeset must be added.

[alionazherdetska]

Closed as a duplicate of: #6201