Minimum version of Eigen

[Lucas-Haubert]

Following the README for installation of Eigen:
"the Eigen C++ template library - conda-forge | repo",
it comes that the suggested version is 3.4.

Moreover, we have guards on version 3.3, e.g.:

#if EIGEN_VERSION_AT_LEAST(3, 3, 90)
      .def(
          "rankUpdate",
          [](Chol &c, const VectorType &w, Scalar sigma) -> Chol & {
            return c.rankUpdate(w, sigma);
          },
          "If LL^* = A, then it becomes A + sigma * v v^*", "w"_a, "sigma"_a,
          nb::rv_policy::reference)
#else

Then should we remove them ?

[ManifoldFR]

Probably yes, since Ubuntu 20.04 (and with it Eigen 3.3.7, the last 3.3.x release) are end-of-life

Do you concur @jorisv @nim65s ?

[nim65s]

lgtm