[TMVA] Enforce supported version of TensorFlow

[vepadulano]

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

[guitargeek]

Thanks!

An important link is also the release notes of TensorFlow, which indicate that there was a switch from Keras 2 to Keras 3:
https://blog.tensorflow.org/2024/03/whats-new-in-tensorflow-216.html

[guitargeek]

I see these build failures on alma9:

  Scanning dependencies of target G__TMVAUtils
  Consolidate compiler generated dependencies of target G__TMVAUtils
  [ 98%] Building CXX object tmva/tmva/CMakeFiles/G__TMVAUtils.dir/G__TMVAUtils.cxx.o
  [ 98%] Linking CXX executable TestRModelParserPyTorch
  /usr/bin/ld: cannot find -lblas

But these problems are not related to this PR, which only changes the requirements.txt.

[github-actions]

Test Results

    10 files      10 suites   2d 1h 26m 13s ⏱️
 2 628 tests  2 583 ✅ 0 💤  45 ❌
24 638 runs  24 506 ✅ 2 💤 130 ❌

For more details on these failures, see this check.

Results for commit e1166c0.

[guitargeek]

Merging now, so I can make a bulk backport of the recent TMVA build fixes to the other branches