custom-resource-existing-cup needs upgrade

[pareis]

Is there an existing issue for this?

• I have searched existing issues, it hasn't been reported yet

Use case description

I've found the lambda that is created when using existing = true, see here: https://forum.serverless.com/t/function-s3-trigger-with-existing-true-generates-another-lambda-called-custom-resources/9440
is using the soon to be sunset node16.x runtime. This triggers AWS to send warning emails regularly. We can not currently control what runtime it uses.

Proposed solution (optional)

I'm thinking:

1. the runtime should be upgraded to node18.x or node20.x, or,
2. we should get control over this by selecting the existingRuntime

[joseph-so]

This one is related to #12133