Update TensorFlow (optional) dependency version specification.

Set minimal version to 2.11.0 (rather than 2.5.0):
- 'TensorflowOptiModule' adapter is based on the "new" Optimizer API
  introduced with TensorFlow/Keras v2.11, but the required dependency
  version had not been updated until now.

Set maximal version to 2.15.X:
- TensorFlow 2.16 (the first release candidate of which was added to
  PyPI three days ago) is switching to Keras 3 by default, which is
  bound to introduce breaking changes (current unit tests fail with
  'tensorflow==2.16.0rc0').
- For now, the maximal version is thus set to 2.15.X. This may change
  in the near future when some time and effort is put in understanding
  changes and/or configuring dependencies to keep using Keras 2 rather
  than Keras 3 in TensorFlow's backend.