Add pre-validation step for BraTS 2023 evaluation wf

[vpchung]

This year, we are asking participants to make predictions using the label "3" instead of "4". To help ease participants who are more familiar with previous BraTS challenges (where "4" was used), Spyros suggests to add a pre-validation check to the workflow that will update all "4" labels to "3".

CaPTk has a CLI tool that will achieve this ask.