Security-iap sample not working properly

Sample not working properly for main branch, possibly for all 4.x versions.

when accessing `curl localhost:8080/` or `curl localhost:8080/topsecret` both return nothing and no error either. Different than the expected behavior in [readme](https://github.com/GoogleCloudPlatform/spring-cloud-gcp/blob/d3b5e4f4e0c6d7c760ef291ce4c940fbd6494385/spring-cloud-gcp-samples/spring-cloud-gcp-security-iap-sample/README.adoc?plain=1#LL54C1-L63C29).

- `curl localhost:8080/` is just a base line comparison defined [here](https://github.com/GoogleCloudPlatform/spring-cloud-gcp/blob/7bfddf3539d7fda04a79fb193eecc39bf6c14a34/spring-cloud-gcp-samples/spring-cloud-gcp-security-iap-sample/src/main/java/com/example/ExampleController.java#L47-L50). Suspect some change is in conflict with `spring-boot-starter-web`

This sample does not have an associated test, to find out when this started to break, tested run:
- Not working: v4.0.0, v4.3.0
- Working: v3.1.0, v3.5.0

Start with spring-cloud-gcp-samples/spring-cloud-gcp-security-iap-sample/README.adoc and src/main/java/com/example/ExampleController.java, then reproduce the reported curl requests on the main branch. Compare the behavior with versions 3.1.0, 3.5.0, 4.0.0, and 4.3.0; done means the sample matches the README's expected responses for both endpoints and has coverage for the regression.

Written by the indexing model from the issue text.