Change default APM server_url to 'http://127.0.0.1:8200' to avoid IPv6 ambiguity

# Description

The current default APM server URL -- 'http://localhost:8200/' -- is ambiguous. "localhost" can resolve to a '127.0.0.1' (IPv4) or '[::1]' (IPv6), or possibly other custom-added IP addresses. The [spec has been changed](https://github.com/elastic/apm/pull/727) to require `http://127.0.0.1:8200` be the default `server_url`.

## Agent Issues

- [x] https://github.com/elastic/apm-agent-nodejs/pull/3049
- [ ] https://github.com/elastic/apm-agent-java/issues/2925
- [ ] https://github.com/elastic/apm-agent-dotnet/issues/1951
- [ ] https://github.com/elastic/apm-agent-python/issues/1712
- [ ] https://github.com/elastic/apm-agent-go/issues/1362
- [ ] https://github.com/elastic/apm-agent-php/issues/838
- [ ] https://github.com/elastic/apm-agent-ruby/issues/1332

## Other places to update

(Not sure if I should make separate issues for each of these.)

- [ ] Kibana: https://github.com/search?q=repo%3Aelastic%2Fkibana+%22localhost%3A8200%22&type=code
- [ ] Hits for 'localhost:8200' in these repos:
```
1 apm
1 apm-contrib
1 hey-apm
1 kibana-load-testing
1 observability-test-environments
2 apm-dev
2 elasticsearch-py
2 opbeans-dotnet
3 apm-aws-lambda
3 observability-dev
4 apm-integration-testing
4 opbeans-node
8 observability-docs
```