Use eigen logistic function for inv_logit

[jachymb]

There is the following comment int prim/fun/inv_logit.hpp from Jan 8 2020

// TODO(Tadej): Eigen is introducing their implementation logistic() of this
// in 3.4. Use that once we switch to Eigen 3.4

We use Eigen 3.4 since Mar 10 2023 and there is in fact a logistic function available: https://eigen.tuxfamily.org/dox/namespaceEigen.html#title47

so this would simplify code and perhaps perform a bit better?