Maintenance: Limit end-to-end tests concurrency

### Summary

Limit e2e tests concurrency to 1

### Why is this needed?

If multiple e2e test workflows run in parallel they can interfere with the infrastructure and fail.

### Which area does this relate to?

_No response_

### Solution

_No response_

### Acknowledgment

- [x] This request meets [Powertools for AWS Lambda (.NET) Tenets](https://docs.powertools.aws.dev/lambda/dotnet/#tenets)