Bump tensorflow to 2.12

[ascillitoe]

2.12 release notes here.

Possibly important changes:

• Python 3.11 added and 3.7 dropped. - No action needed. We are tracking upgrade to Python 3.11 in Support Python 3.11 #676, and 3.7 will be dropped when EOL
• Moved all saving-related utilities to a new namespace, keras.saving, for example: keras.saving.load_model, keras.saving.save_model, keras.saving.custom_object_scope, keras.saving.get_custom_objects, keras.saving.register_keras_serializable,keras.saving.get_registered_name and keras.saving.get_registered_object. The previous API locations (in keras.utils and keras.models) will be available indefinitely, but we recommend you update your code to point to the new API locations. - We could change tf.keras.models.load_model used in saving._tensorflow.loading to tf.keras.saving.load_model, but would need logic to cope with older tf versions, and old api locations available indefinitely, so best to leave.
• The new Keras model saving format (.keras) is available. You can start using it via model.save(f"{fname}.keras", save_format="keras_v3"). In the future it will become the default for all files with the .keras extension.... - We could explore moving to keras_v3 instead of SavedModel, but would not be compatible with older tf versions so probably best to leave...

[codecov-commenter]

Codecov Report

Merging #764 (fdba5f5) into master (3bd3148) will decrease coverage by 0.05%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #764      +/-   ##
==========================================
- Coverage   80.86%   80.82%   -0.05%     
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  Files         144      144              
  Lines        9648     9607      -41     
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- Hits         7802     7765      -37     
+ Misses       1846     1842       -4     

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ubuntu-latest-3.8	80.70% <ø> (?)
ubuntu-latest-3.9	80.70% <ø> (-0.05%)	⬇️

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[ascillitoe]

@jklaise @mauicv CI is now passing. I've put down my thoughts above on what I see as the main changes of interest. To me it looks like we are good to merge (same for alibi).