fix(nextjs): Remove sentry property from returned next config

[mitchheddles]

Next recently added config validation vercel/next.js#38498 so withSentryConfig should remove the additional sentry property to avoid these warnings.

Fixes#5449

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[lobsterkatie]

Hi, @mitchheddles.

Thanks for the contribution!

This is a good idea, but you'll see that some of your tests are failing. This is because of a timing issue, even though I see that you moved constructWebpackConfigFunction to run before the deletion. The problem is that it's not actually this function that needs the sentry data, but rather the function it returns. Since that function doesn't run until later in the build process, by the time it's looking for the data, it's already been deleted.

The fix is to grab it and store it in a closure around the returned function. I tried to make those changes in this PR, but got an auth error when I tried to push. I've therefore created a new PR combining your original idea and the fix: #5473.

Cheers!