Bump MOZCI_VERSION to 4

Since mozci 1.6.0 introduces some important changes to regression detection.

scripts/test_scheduling_history_retriever.py

@@ -29,7 +29,7 @@ logger = getLogger(__name__)
 # The mozci version (to bump whenever we change the mozci regression algorithm),
 # so we can keep track of which version of mozci was used to analyze a given push
 # and we can decide when we want to regenerate parts of the dataset.
-MOZCI_VERSION = 3
+MOZCI_VERSION = 4
 
 TRAINING_MONTHS = {
     "label": 7,