Review & upgrade deprecated features/versions

[micya]

Since the Notification System was built on Azure Functions ~5 years ago, various features have been deprecated or reached end of life. Check which features need to be upgraded. For example:

• Cosmos DB extension reaching EOL in Aug 2024: https://learn.microsoft.com/en-us/azure/azure-functions/migrate-cosmos-db-version-3-version-4
• .NET 6 reaching EOL in Nov 2024: https://dotnet.microsoft.com/en-us/platform/support/policy
• Some kind of runtime deprecation: https://learn.microsoft.com/en-us/azure/azure-functions/migrate-dotnet-to-isolated-model
• App Insights new version: https://learn.microsoft.com/en-us/azure/azure-monitor/app/convert-classic-resource

This is not an exhaustive list - likely there are other features which I have missed. An assessment should be made for all Azure services used.