Add deprecation warning for TensorFlow backend (#3734)

### Reason for changes

Removing support TF backend in the next support

Author: AlexanderDokuchaev

src/nncf/tensorflow/__init__.py

@@ -12,6 +12,7 @@
 Base subpackage for NNCF TensorFlow functionality.
 """
 
+from nncf.common.deprecation import warning_deprecated
 import tensorflow
 from packaging import version
 from packaging.specifiers import SpecifierSet
@@ -54,3 +55,7 @@
 from nncf.tensorflow.sparsity.magnitude import algorithm as magnitude_sparsity_algorithm
 from nncf.tensorflow.sparsity.rb import algorithm as rb_sparsity_algorithm
 from nncf.tensorflow.utils.state import ConfigState
+
+warning_deprecated(
+    "The TensorFlow backend is deprecated and will be removed in a future release."
+)