Remove Next.js telemetry

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.
![grafik](