// step 6: concatenate everything into a single feature column
.Append(mlContext.Transforms.Concatenate(
    "Features",
    "Age",
    "Pclass",
    "SibSp",
    "Parch",
    "Sex",
    "Embarked"))

// step 7: use a fasttree trainer
.Append(mlContext.BinaryClassification.Trainers.FastTree(
    labelColumnName: DefaultColumnNames.Label,
    featureColumnName: DefaultColumnNames.Features));

// the rest of the code goes here...