build: support Eigen 5 (#6036)

* build: support Eigen 5

fix #6034

* build: probe Eigen 3 and 5 separately in CMake config mode

Avoid relying on package-specific handling of a bounded version range when discovering Eigen through Eigen3Config.cmake.

Made-with: Cursor

* [skip ci] build: clarify Eigen 5 module fallback comment

Explain that the MODULE-mode fallback only exists for older Eigen 3 setups so the remaining fallback path does not look like an unresolved Eigen 5 issue.

Made-with: Cursor

* [skip ci] docs: add Eigen 5 entry to v3.0.4 changelog

Document the Eigen 5 CMake package detection fix in the 3.0.4 release notes before merging the PR.

Made-with: Cursor

---------

Co-authored-by: Eisuke Kawashima <e-kwsm@users.noreply.github.com>
Co-authored-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>