Remove Next.js telemetry

[MagicLike]

As seen in the build logs down below, Next.js collects telemetry from the build process.
For privacy reasons umami should opt-out of this by default, as described here.
I have already tested the environment variable in Vercel and can confirm that the telemetry is not shown in the logs. Nevertheless, I would prefer to integrate the opt-out directly into the code, as not everyone thinks about using environment variables.