[BUG] in sample: stripe (FUNCTION_NAME)

### Which sample has a bug?

[Create Stripe customers and charge them on Firestore write](https://github.com/firebase/functions-samples/tree/main/stripe)

### How to reproduce the issue

1. Deploy code with node runtime later than node 8
2. Execute code in a manner that will log an error
3. Observe that "FUNCTION_NAME" will be undefined

Why? Because once cloud functions moved beyond Node 8, the environment variables that are exposed changed. This should now be `FUNCTION_TARGET` as defined here: https://cloud.google.com/functions/docs/configuring/env-var#newer_runtimes

### Debug output

**Screenshots**


### Expected behavior

Expect the cloud function name to be shown, but `FUNCTION_NAME` is not a valid env var after node 8.

### Actual behavior

The name of the cloud function is `undefined`