[fs-detectors] Exclude the middleware builder if it's a Next.js app

[nuta]

While maybeGetApiBuilder() excludes the middleware builder, however, we need another check for the case where projectSettings.framework == null.

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[styfle]

This should take into account the project creation date

vercel/packages/fs-detectors/src/detect-builders.ts

Line 497 in 181a492

(framework === undefined || createdAt < Date.parse('2020-03-01'))

You might even use detectFrontBuilder() instead

[nuta]

I'm seeing failures in CI but it appears the same happen in some other PRs. Are they just flaky?

[styfle]

Thanks!