Enable flag eval metrics tests for .NET v3.43.0-dev

Update manifest to run test_flag_eval_metrics.py tests when using
dd-trace-dotnet v3.43.0 or later (including dev versions).

Author: sameerank

manifests/dotnet.yml

@@ -698,7 +698,7 @@ manifest:
   tests/docker_ssi/test_docker_ssi_appsec.py::TestDockerSSIAppsecFeatures::test_telemetry_source_ssi: v3.36.0
   tests/ffe/test_dynamic_evaluation.py: v3.36.0
   tests/ffe/test_exposures.py: v3.36.0
-  tests/ffe/test_flag_eval_metrics.py: missing_feature
+  tests/ffe/test_flag_eval_metrics.py: v3.42.0-dev
   tests/integration_frameworks/llm/anthropic/test_anthropic_llmobs.py::TestAnthropicLlmObsMessages::test_create_error: bug (MLOB-1234)
   tests/integrations/crossed_integrations/test_kafka.py::Test_Kafka: v2.0.0-prerelease
   tests/integrations/crossed_integrations/test_kinesis.py::Test_Kinesis_PROPAGATION_VIA_MESSAGE_ATTRIBUTES: missing_feature