Arc-enabled Kubernetes MicroHack: replace outdated Defender for Kubernetes naming #599

Description

The Arc-enabled Kubernetes MicroHack still refers to Microsoft Defender for Kubernetes, while the current name is Microsoft Defender for Containers.

For example, Challenge 02 states:

Onboard the cluster to Microsoft Defender for Kubernetes to activate runtime threat detection and security posture management.

The learning link in the same challenge already points to the Defender for Containers documentation:
https://learn.microsoft.com/en-us/azure/defender-for-cloud/defender-for-containers-arc-enable-programmatically

The Challenge 02 walkthrough also already uses Defender for Containers.

Affected locations

• MicroHack README, learning objectives
• Challenge 02: introduction (line 5), onboarding task (line 20), and success criteria (line 25).

Expected change

Replace the outdated Defender for Kubernetes references with Microsoft Defender for Containers throughout this MicroHack, keeping the challenge, README, walkthrough, and linked documentation consistent.