migration plan and guide from classic agent

[trentm]

Some customers will migrate from the classic agent (elastic-apm-node) and should have a guide on how to do so. This should cover:

• equivalents (or close) for all classic agent config options, or a list of limitations and descriptions of impact
• a comparison of which supported-tech is not currently supported in the new distro that is in https://www.elastic.co/guide/en/apm/agent/nodejs/current/supported-technologies.html This is mostly about instrumentations, but should also include things like Node.js versions (if different); diff in ESM support; special cases like Azure Functions, Lambda.
• (stretch) some discussion of instrumentation differences for each significant instr

Part of preparing for this guide should result in issues being created for feature work that we can do to ease the migration.

[estolfo]

This blog post about the Go agent might be a useful reference.