Replace Spring logo with .NET logo in the architecture diagram

Nobody has claimed this yet.

## Description

The repository is a **.NET / ASP.NET Core** workshop, but the base architecture image still carries the **Spring** logo inherited from the original Java workshop.

The affected asset is `assets/order-service-ecosystem.png`, referenced in [step2-exploring-the-app.md](step2-exploring-the-app.md):

```markdown
```

In this diagram, the `Order Service` box displays the green Spring leaf logo, which is misleading for a .NET audience.

## Expected behavior

The diagram should display the **.NET logo** instead of the Spring logo, so the visual identity matches the actual stack used throughout the workshop (ASP.NET Core Minimal API, `Confluent.Kafka`, Testcontainers, Microcks).

## Suggested change

- Replace the Spring leaf logo with the .NET logo in `assets/order-service-ecosystem.png`.
- Keep the rest of the diagram (Order API consumers, Pastry API Client, Kafka topics `orders-created` / `orders-reviewed`, Event Publisher/Consumer) unchanged.
- Verify no other asset under `assets/` still references Spring branding.

## Additional note

There is also a leftover `Spring Test` mention in [step4-write-rest-tests.md](step4-write-rest-tests.md) (line ~338) that could be aligned to .NET wording as a follow-up.

Update assets/order-service-ecosystem.png, which is referenced from step2-exploring-the-app.md. Start by inspecting the diagram and the other files under assets/ for Spring branding. Done means the Order Service box uses the .NET logo, the rest of the diagram is unchanged, and no other asset still references Spring branding.

Written by the indexing model from the issue text.