Add Linux kernel 5.8+ as minimum version requirement for GPU Monitoring (DataDog#37244)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: gouri-yerra

content/en/gpu_monitoring/setup.md

@@ -28,6 +28,7 @@ To begin using Datadog's GPU Monitoring, your environment must meet the followin
 
 - **Datadog Agent**: v7.74
 - **Operating system**: Linux
+- **Linux kernel**: 5.8 and above
 - **NVIDIA driver**: version 450.51
 
 If using Kubernetes, the following additional requirements must be met: