Fix jax version to 0.6.0 in dependencies [TEMPORARY FIX]

[Giulero]

This PR provides a temporary fix for the recent jax update from version 0.6.0 to 0.7.0.
See #132 and #131.

For now, I'm fixing the JAX version in the dependencies to the last working version.
I'll fix the general issue hopefully soon.

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Copilot summary

This pull request updates the way the jax dependency is specified in both the ci_env.yml and pyproject.toml files. The main change is to pin the version of jax to 0.6.0 instead of allowing any version, which helps ensure consistent environments and reduces the risk of incompatibility issues.

Dependency version pinning:

• Updated the ci_env.yml file to specify jax==0.6.0 instead of an unpinned version for more predictable CI builds.
• Modified the pyproject.toml file to pin jax to version 0.6.0 in both the jax optional dependency group and the test dependency group, improving reproducibility for development and testing.

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📚 Documentation preview 📚: https://adam-docs--133.org.readthedocs.build/en/133/

[Giulero]

Thanks @GiulioRomualdi!

[traversaro]

Can you please use an upper bound in pyproject.toml instead of an equality constraint? If earlier jax version worked fine, I see no reason to forcing users to use only a specific Jax version.

[Giulero]

Can you please use an upper bound in pyproject.toml instead of an equality constraint? If earlier jax version worked fine, I see no reason to forcing users to use only a specific Jax version.

Super true! Thanks @traversaro for the suggestion!