Default behaviour should be to not automatically set any Lambda environment variables

[neverendingqs]

Right now, all Lambda function environments are configured with all variables in the dotenv file(s). This can easily cause security issues by setting environment variables that contain secrets (e.g. SENTRY_SECRET gets set when none of the Lambda functions use it). As well, it can cause confusion on what is happening on different environments (e.g. #65).

The new default value should be functionally equivalent to:

custom:
  dotenv:
    include: []

• Update README to recommend new safe default
• Update README to warn about deprecation
• Add or update toggle to switch behaviour on major version bump