update Lambda guide to point out ELASTIC_APM_API_KEY for Elastic serverless usage

[trentm]

IIUC, the new Elastic cloud "serverless" projects only support API key auth and not the "Bearer $secretToken" auth. The Lambda setup guide (https://www.elastic.co/guide/en/apm/agent/nodejs/current/lambda.html) favours ELASTIC_APM_SECRET_TOKEN and only mentions using API key "if you prefer". Those docs should be updated to state more clearly when API key maybe required.

(We should also check the other APM agent Lambda guides for the same.)