Support Eigen 5.0.0

[jwnimmer-tri]

Is your feature request related to a problem? Please describe.

See https://gitlab.com/libeigen/eigen/-/releases/5.0.0.

Describe the solution you'd like

Switch our wheel builds (via BCR) to use Eigen 5.0.0 so we have CI coverage that it also works (alongside our existing support for 3.4.0, which will remain intact).

[jwnimmer-tri]

See my eigen-5 branch for work in progress, 99% finished.

There is a bug in Eigen (ignoring binary operator traits for Monomial op Monomial => Polynomial) that we'll either need to patch upstream and wait for the next release, or find some complicated work-around for.