Modify CosmosDB client to use `System.Text.Json`

[SebastianKuesters]

Right now we are using Cosmos DB SDK version 3.x, which is using Newtonsoft.Json internally. This is a problem for us, as we want to use System.Text.Json everywhere.

The switch to System.Text.Json is on roadmap for version 4.x of the SDK since years: Azure/azure-cosmos-dotnet-v3#2533

Let's try out this workaround here to switch to System.Text.Json
https://ankitvijay.net/2021/06/20/custom-json-serializer-settings-with-cosmos-db-sdk/