Downgrade DSM flush shutdown log from Error to Warning (#7984)

## Summary of changes
Addressing some [test
flake](https://dev.azure.com/datadoghq/dd-trace-dotnet/_build/results?buildId=192892&view=logs&j=31acdf08-3064-571d-e44c-78b15fe9b1f9&t=582a4466-53a3-57ed-5676-bdeb8e6ba7e6).
This can happen in an overloaded CI environment.

## Reason for change

## Implementation details

## Test coverage

## Other details
<!-- Fixes #{issue} -->


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Author: robcarlan-datadog

tracer/src/Datadog.Trace/DataStreamsMonitoring/DataStreamsWriter.cs

@@ -184,7 +184,7 @@ private async Task FlushAndCloseAsync()
 
         if (completedTask != allTasks)
         {
-            Log.Error("Could not flush all data streams stats before process exit");
+            Log.Warning("Could not flush all data streams stats before process exit");
         }
 
         await FlushAsync().ConfigureAwait(false);