get Next.js instrumentation to GA

In https://github.com/elastic/apm-agent-nodejs/pull/2959 we plan to have a technical preview of Next.js instrumentation. It'll be on by default. We should get this instrumentation to "GA" (i.e. dropping the technical preview nomenclature) at some point. Here is a list of proposed items to do:

- [ ] Get some engagement with / validation of the current instrumentation with some Next.js users.
- [ ] Investigate RUM agent integration and doc that setup / best practices for users; without repeating content that already exists for the RUM agent.
- [ ] Sanity test that a TypeScript Next.js app is instrumented properly (no reason it shouldn't be, I just haven't tested).
- [ ] See if a merge of the tech. preview `-r elastic-apm-node/start-next.js` to the common `-r elastic-apm-node/start.js` would be feasible.
- [ ] Restore Windows testing in next.test.js, which is currently being skipped.

And perhaps other things will come up with some user feedback.