[BUG]: does not work with Eigen 5

### Required prerequisites

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### What version (or hash if on master) of pybind11 are you using?

98e50b702d9ab784ec918399aaa20adcfd1af9a9

### Problem description

Eigen 5 was released on 30th Sep 2025 <https://gitlab.com/libeigen/eigen/-/releases/5.0.0>.

The following macros are no longer available:

https://github.com/pybind/pybind11/blob/98e50b702d9ab784ec918399aaa20adcfd1af9a9/tools/FindEigen3.cmake#L36-L44

namely version detection fails.

### Reproducible example code

```text
$ cmake -S . -B build
…
-- Eigen3 version .. found in /usr/include/eigen3, but at least version 3.2.7 is required
-- Building tests WITHOUT Eigen, use -DDOWNLOAD_EIGEN=ON to download
…
```

### Is this a regression? Put the last known working version here if it is.

Not a regression

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