### What?
Let Next.js enable React Strict Mode by default for the App Router.
This caught a bug with the `AppRouterAnnouncer` added in #47018 which
should be fixed as well.
### Why?
This logic:
a0d1d728b9/packages/next/src/build/webpack-config.ts (L326-L333)
assumes that the default value of `reactStrictMode` is `null`, but it
was set to `false` in #46656.
### How?
This PR reverts the default value to `null`.
Closes NEXT-1484
Fixes#53315
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Co-authored-by: Tim Neutkens <tim@timneutkens.nl>
96a59344e8