chore(deps): allow anesthetic>=2.9.0 to unblock jax>=0.7 / numpy>=2 resolution

[Jammy2211]

Summary

• Bumps anesthetic from the exact pin ==2.8.14 (python<3.13 branch) to >=2.9.0, collapsing the previous two python-version-conditional lines into one.
• The old pin transitively required numpy<2, which caused pip's resolver in release.yml's until ... pip install autoconf[optional]==$VERSION ... block to backsolve jax to 0.4.28. Subsequent steps then silently dropped JAX from the environment entirely, so every run_scripts job failed in ~0.1s with ModuleNotFoundError: No module named 'jax'.
• After this bump, pip install -e .[optional] locally resolves to anesthetic 2.14.6, numpy 2.2.6, jax 0.9.2.

Why this is safe

• anesthetic>=2.9.0 supports Python 3.9 (the existing python_version<'3.13' branch's lower bound), so the conditional pin is no longer needed.
• The new >= constraint matches what was already in place for python>=3.13.
• The nss/blackjax compatibility note in the file is updated: the previous "bump only after both pins move together" constraint is now resolved.

Test plan

• Local pip install -e .[optional] resolves jax 0.7+ (got 0.9.2)
• pytest test_autofit — 1404 passed, 13 failed (all pre-existing NSS tests requiring the manually-installed handley-lab/blackjax fork via pip install autofit[nss], unrelated to anesthetic)
• CI on this PR
• Paired with PyAutoBuild release.yml backstop PR before next pre_build dispatch

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