Significant update - latest packages and libraries, multiple changes

[MarkWme]

Making multiple changes to support the latest packages. Significant updates to Langchain, Azure AI Search and Semantic Kernel have made it necessary to make many changes.

Summary list

• Changing Azure Cognitive Search to Azure AI Search in several places
• Changed environment variable names to match product branding changes and to work better as default values for some libraries
• Python packages in requirements.txt updated to latest versions (as at 22nd January 2024)
• .NET packages updated to latest versions (as at 22nd January 2024)
• Most labs required significant updates in order to work correctly with the most up to date packages
• The Azure Search labs were significantly re-worked to make them easier to understand
• The Semantic Kernel / Python version of the Azure Search lab was temporarily removed until the Semantic Kernel Python packages stabilise

In short, test everything. Most things changed!

[denniszielke]

LGTM