cap tensorflow version to <2.12.0

Cap tensorflow version to <2.12.0 until we have time to adapt to the API changes such as e.g.
* Support for Python 3.11 has been added.
* Support for Python 3.7 has been removed. 
See also https://github.com/tensorflow/tensorflow/releases/tag/v2.12.0.

requirements.txt

@@ -1,7 +1,7 @@
 # ocrd includes opencv, numpy, shapely, click
 ocrd >= 2.23.3
 scikit-learn >= 0.23.2
-tensorflow >= 2.4.0
+tensorflow >= 2.4.0, <2.12.0
 imutils >= 0.5.3
 matplotlib
 setuptools >= 50