[tmva][sofie] Don't run Keras tests if keras>=3.5

This is a followup to 10f28b6, where I used a random keras version
in a check in order to make the CI configuration at the time pass. In
fact, `keras=>3.5` also didn't work, as we see now after updating the
macOS runners from Keras 2 to Keras 3.

It could actually turn out that `keras>=3` is not supported at all, so
we should be prepared to lower the maximum supported Keras version even
further. But for now we don't know, as no platform has
`keras>=3.0&&keras<3.5` installed.

Author: guitargeek

tmva/sofie/test/CMakeLists.txt

@@ -154,7 +154,7 @@ endif()
 # Any reatures that link against libpython are disabled if built with tpython=OFF
 if (tpython AND ROOT_KERAS_FOUND AND BLAS_FOUND)
 
-  set(unsupported_keras_version "3.10.0")
+  set(unsupported_keras_version "3.5.0")
 
   # TODO: make sure we also support the newest Keras
   if (NOT DEFINED ROOT_KERAS_VERSION)