Update `blog-starter` to App Router (#61170)
### What?
This updates the `blog-starter` example to Next 14.1 App Router.
### Why?
I checked out a new `blog-starter` project on Vercel and was surprised
it was using the Pages Router. I believe the App Router is a better
choice.
### How?
I tried to keep the implementation logic as close to the original Pages
`blog-starter`.
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Co-authored-by: Sam Ko <sam@vercel.com>
