docs(ppr): remove v14 mention for ppr (#71498)
## Why?
For PPR (experimental), user's should just opt to use v15 canary if they
want to try out PPR. Removing the v14 mention reduces this confusion.
- x-ref:
https://vercel.slack.com/archives/C05KYT5S9FF/p1729264052080029?thread_ts=1729263181.989139&cid=C05KYT5S9FF
