Add Tensorflow support for variable scatter_update in optimizers.

[hertschuh]

This should have been added along with scatter_add and scatter_sub as part of #18692 as it was added in the super class: https://github.com/keras-team/keras/blob/master/keras/optimizers/base_optimizer.py#L222

Fixes #19281

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 54.45%. Comparing base (c8700f4) to head (bb9551b).
Report is 98 commits behind head on master.

Files	Patch %	Lines
keras/backend/tensorflow/optimizer.py	0.00%	7 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #19313       +/-   ##
===========================================
- Coverage   80.14%   54.45%   -25.69%     
===========================================
  Files         341      366       +25     
  Lines       36163    40078     +3915     
  Branches     7116     7771      +655     
===========================================
- Hits        28982    21825     -7157     
- Misses       5578    16662    +11084     
+ Partials     1603     1591       -12     

Flag	Coverage Δ
keras	54.45% <0.00%> (-25.54%)	⬇️
keras-jax	?
keras-numpy	54.45% <0.00%> (-2.63%)	⬇️
keras-tensorflow	?
keras-torch	?

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[fchollet]

Thanks for the fix!