Upgrade Next.js to v15

[Tyratox]

After some testing, most of the applications seems to be working. This matches the migration guide at https://nextjs.org/blog/next-15 according to which there are not too many surprises for this project.

However, we need to hold back the upgrade for the moment because storybook crashes at the moment with the error "Cannot render react components created by older version".

May be related to storybookjs/storybook#29421. And most likely we need to wait for an update of https://www.npmjs.com/package/@storybook/nextjs?activeTab=readme.