[formrecognizer] use value_selection_mark once service returns this value

[kristapratico]

Known service bug causing value_selection_mark to be None on a FieldValue of type == "selectionMark". We are working around this by returning the text value in the SDK: https://github.com/Azure/azure-sdk-for-python/blob/master/sdk/formrecognizer/azure-ai-formrecognizer/azure/ai/formrecognizer/_models.py#L67

ETA v2.1 GA for fix

[kristapratico]

Updated to June milestone, fix will be in GA release