standardize term for `durable functions backend`

Align on succinct name for the Lambda Service to use throughout, and standardize on that name.

Current docs generally use "Lambda" or "Lambda Service", but this results in

> When it's time to resume, Lambda invokes your function again... multiple invocations of a Lambda function...

(from
https://docs.aws.amazon.com/lambda/latest/dg/durable-basic-concepts.html)

"Lambda" or "Lambda service" collides with the end-user's concept of their Lambda function (i.e user space).

_Originally posted by @dhegberg in https://github.com/aws/aws-durable-execution-docs/pull/118#discussion_r3053476119_