Fix keystone dev errors for next@^13.4.0

[MohammadKurjieh]

This resolves: #8553

On next version 13.4.0 and higher setting the appDir to false is mandatory in the next config file.
Ref: vercel/next.js#49261 and vercel/next.js#49251

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[dcousens]

I wonder, won't this flag disappear in an upcoming next patch release? 🤔

[dcousens]

LGTM, but if experimental.appDir drops in an upcoming next version, we need to resolve this differently.
Users might actually want appDir: true too (though, this might not affect them if they are doing their own next app)

[Thinkscape]

LGTM, but if experimental.appDir drops in an upcoming next version, we need to resolve this differently. Users might actually want appDir: true too (though, this might not affect them if they are doing their own next app)

But this is only for the admin ui, which means that it only affects users that want to use appDir inside the /admin/ folder. Probably the complete fix would be converting the current /admin/pages* generators to app router.

[dcousens]

Until we have an alternative solution, this is good to go, thanks @MohammadKurjieh