Update OpenVINO notebooks for Ultralytics YOLOv8.1 release

[glenn-jocher]

@adrianboguszewski this PR updates all 3 OpenVINO notebooks for YOLOv8.1 usage. YOLO8.1 is our most recent form January 10th, 2024: https://github.com/ultralytics/ultralytics/releases/tag/v8.1.0

The main changes are:

• Update notebook installation steps from ultralytics==8.0.143 to ultralytics>=8.1.
• Update import structure to reflect newer simplified usage with removal of ultralytics.yolo module, i.e. ultralytics.yolo.utils is now ultralytics.utils, and the same for other imports that used to be under ultralytics.yolo. This modification reflects an emphasis on cross-model compatibility for the ultralytics package.
• Minor README updates.