Migrate to Next 15

[bambooch]

I just found out that Next 15 RC2 came out yesterday so it might be cool to play around with the newest festures. Migrating shouldn't be too big of an issue since we, afaik, don't use the affected APIs. This is of course open to discussion on whther it should or should not be developed.

https://nextjs.org/blog/next-15-rc2?ref=dailydev#smooth-upgrades-with-codemod-cli