Bump minimum numpyro version to >= 0.21.0 and remove all tiny float padding for zero Poisson and Negative Binomial means

[dylanhmorris]

NumPyro >=0.21.0 supports 0-mean Poisson and Negative Binomial distributions. We should bump our minimum version and remove the use of epsilon padding.

NumPyro >=0.21.0 also addresses incompatibilities with the latest JAX, so we can also unpin and bump JAX in this PR.