Use the exising breakpoints utlity in the new composible header

[oliverschuerch]

In the new composible header component we use a custom breakpoint detection mechanism.
But there is a breakpoints utility which should be used, so we can reduce code, improve page performance and maintain the detection mechanism from a single source of truth.

Have a look into the utility: https://github.com/swisspost/design-system/blob/main/packages/components/src/utils/breakpoints.ts
And how it is used in the post-footer component: https://github.com/swisspost/design-system/blob/main/packages/components/src/components/post-footer/post-footer.tsx

[alionazherdetska]

After discussion in the dev roundtable, we've decided to close this PR and create a new ticket for the breakpoints utility implementation in the post-header component #5357.

Reasons:
This PR has been on hold for an extended period
There are now several merge conflicts that would be time-consuming to resolve
It would be more efficient to implement this from scratch with the current codebase