Update build worker warning to use debug (#60847)
Since we are enabling `webpackBuildWorker` by default now we should move
this warning to debug since it can be confusing without additional
context.
x-ref: [slack
thread](https://vercel.slack.com/archives/C04KC8A53T7/p1705612748627769?thread_ts=1705599489.811039&cid=C04KC8A53T7)
Closes NEXT-2158
