nextjs package: build error because of a post-accordion element without the required heading-level property

[oliverschuerch]

Describe the bug
The post-accordion web-component now defines its headingLevel property as required!
Therefore we get a error in the nextjs package (only used for testing at the moment), because in the page.tsx file such a component is used, but without this attribute.

To Reproduce

1. checkout the main branch
2. run ``pnpm bootstrap`

Expected behavior
To have no error when building the nextjs package

Screenshots
If applicable, add screenshots to help explain your problem.

[oliverschuerch]

@alizedebray I hope it is ok that I assigned you to this task directly.
I think most of our PRs are red, because of this and as soon as it is fixed a lot of them should become green again.
If you need someone to approve the PR tomorrow, just login with the oss@post.ch user in a different browser ;)
But don't say that to Philipp 😆