[TMVA] Enforce supported version of TensorFlow

We do not support Tensorflow >2.15 since Keras 3.x brings several breaking changes keras-team/keras#18467

Author: vepadulano

requirements.txt

@@ -5,7 +5,7 @@
 numpy>=1.4.1
 scikit-learn
 xgboost
-tensorflow
+tensorflow<2.16
 torch
 
 # PyROOT: ROOT.Numba.Declare decorator