Rename sharding_names to out_sharding in NNX Variable metadata

This CL renames the sharding_names attribute to out_sharding for better
consistency with the sharding API. The new name more clearly indicates
the purpose of this metadata field.
## Changes
- Bump Flax version to 0.12.4
- Core changes in variablelib.py:
  - Add sharding_names to out_sharding metadata remapping for backward compatibility
  - Add deprecated sharding_names property that returns out_sharding with a warning
- Update nnx/spmd.py, core/spmd.py, core/meta.py, linen/spmd.py to use out_sharding
- Update all NNX tests to use the new attribute name
- Update qwix flax_util.py to check for out_sharding first, with fallback to sharding_names
- Update maxtext initializers.py to check for out_sharding first
- Update documentation and examples to use out_sharding
## Backward Compatibility
Existing code using sharding_names will continue to work via:
- Metadata remapping during Variable creation
- Deprecated Variable.sharding_names property

PiperOrigin-RevId: 869269899

update flax to version 0.12.3

PiperOrigin-RevId: 861815039

Merge pull request #5149 from IvyZX:release

PiperOrigin-RevId: 846407227

Merge pull request #5095 from google:jax-minver-0.8.1

PiperOrigin-RevId: 834401927

Merge pull request #4983 from google:propagate-eager-sharding

PiperOrigin-RevId: 811550398

update version to 0.11.2

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Merge pull request #4878 from google:ver-0.11.1

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Merge pull request #4854 from google:mutable-array-guide

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Merge pull request #4798 from google:ver-0.10.7

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Merge pull request #4724 from google:0.10.6

PiperOrigin-RevId: 750649299