Prevent further issues with Cosmos DB MongoDB partition key overflows

Update 6/6/2024:

In the short-term we are solving this problem here:
- https://github.com/Azure/azure-sdk-tools/issues/8389

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This work item is about making a long-term fix to:
- https://github.com/Azure/azure-sdk-tools/issues/6769

Most likely we want to do two things:

- `Settings -> Features -> Update MongoDB server version`
- Set up [TTL](https://learn.microsoft.com/en-us/azure/cosmos-db/mongodb/time-to-live)
- Note that currently there is no "created at" timestamp at all in the db. See #6769 for heuristics we used to approximate it.
- Figure out a way to delete the duplicated `DocumentDBDefaultIndex` index in `openapiPipeline -> pipelineResults -> Scale & Settings -> Indexing Policy`. Just trying to delete it results in error.
- Try to diagnose the database throughput - currently, trying to delete even 100 docs fails most of the time with timeout.