`CLERK_JS_VERSION` should be `NEXT_PUBLIC_CLERK_JS_VERSION`

If you are using CLERK_JS_VERSION as an environment variable, it should be changed to NEXT_PUBLIC_CLERK_JS_VERSION instead.

This change brings our SDK up to date with the latest standards for next.js - that public environment variables should have the NEXT_PUBLIC_ prefix. This env variable is not private, so it should get the public prefix.